Let me see if I have any knowledge that can help you--in fact I do know something that I have concluded myself from my knowledge of psychology and neuroscience, that nobody else has concluded. You will find that it makes sence, and it might help you with your question. Here goes:
The most important issue for my contribution is this: Is he your First Great Love? I assume the answer is YES; but for How Long were you lovers? Did you break up at the end of high school, when you were 18 and before you'd ever lived together? Do you have a secure feeling of attachment to either your mother or your father? like you know for certain that she or he would always be safe and caring for you no matter what happens? [Believe it or not, there are many people for whom neither parent has continued to be such a reliable "good mother" or "good father."] Did you actually replace your reliable "good parent" safety with your boyfriend, before he left for University?
These questions are vitally important for the insight that I have gained that I now apply to First Great Love relationships, because I'm doubtful whether a First Love of 2 yrs or less that was interrupted at 18 would be strong enough in the brain structures laid down to manage it to become a template/model for all future love relationships, UNLESS one or both partners had NO adequately secure "good parent" template before they got romantically involved.
I will not present my knowledge about that template until I hear your answers to my questions aimed at the power of your prior bond with your boyfriend. I can suggest, however, that your high school sweehearts relationship was not concluded and broken up because of irreconcilable differences and/or intolerable conflicts, so neither of you has a strong reason for NOT being able to restimulate the love feelings you had before he went away to University. So your relationship just went into limbo, and You didn't get around to replacing him in your home town, because you weren't trying to. But he's been in a stimulating new environment where he experiences himself as changing, and he can be attracted to other people in his new (& therefore changing) surroundings. And "new love chases away old love."
But he never denied his past feelings for you; he just put them away on the shelf, with an open door in his mind to go back to you if he should ever want to. So he did. If his loving-feeling, -perceiving and acting patterns with you are firmly established in his brain structures as the "right & authentic form of love-relationship," he will be somewhat confused and uncomfortable, like a fish out of water, with any new love, after the first few months of romantic excitement have passed, that is he'll be confused & uncomfortable with whatever in his new relationship doesn't fit with the patterns formed with you. But if those First Love patterns aren't as hard and fast as First Great Love--which I think needs to be comparable with First Marriage, then it may take him a year or two, but he will be able to form new loving patterns that overwrite the older ones. I have studied over 1500 relationship history papers written by my Psych of Relationship students over the 17 years I was collecting their reflections as the premier paper for my class, and I found that high school First Loves tended to last only into the first year of college away from home, though the partner that stayed at home was less likely than the one at college to initiate a breakup &(because she or he wanted to) start another romance. Though other factors are important too, the first post-breakup relationship is less likely to last than the second, which is likely to come in 2nd, 3rd or 4th year of University. But the experience of growth and excitement at University makes it unlikely that the student will keep returning to the home-town relationship, unless "new loves" are aborted before they pass the 4 month point and grow more deeply into the new intimate territority that's available in the freer intellectual, moral and emotional environment that's no longer leashed to the family of origin.
There are significant gender differences in the balance of emotional closeness between family of origin and romantic lover, with boys being expected and allowed to gain greater freedom from family influence at an earlier age; being away at university only increases that difference. But if you are following an expected path in which you will NOT leave your home town before marriage, but will probably marry a guy who also stays in your home town, then you will benefit from the continuity of family and neighborhood society that people without professional careers enjoy. But unless your boyfriend feels it necessary to return and build his adult life in your home town after University, he is more likely to prefer adjusting his relating habits to someone he meets later on in University, or even later, after he relocates to a place where his professional career has begun. And if your high school sweethearts bond was around 2 years long or less, then your expectations towards marriage and parenting after high school diverged from his expectations of another premarital stage while he's developing his professional career.
If your bonding situation has been more intense and/or enduring than that, then the outcome that you're hoping for might still be likely. By more INTENSE, I mean that your emotional bonding might have been more powerful and potentially enduring, IF you had major emotional upheavals during your relationship, such as parental death or divorce, major physical or emotional scares or conflicts, miraculous experiences, tragedies or spectacular emotional or sexual closeness.
I've written just about all I can without the answers to my questions, in case you'd just rather draw your own conclusions about how strong your precollege bond may be, so that you can assess whether your own expectations are really matched by the physiological and psychological forces at work within each of you. So you can decide whether you want to discuss the matter with my any more or not.
I do hope, however, that my extensive contribution here has shown you that it is possible to get a highly expert answer to your very serious and justified question, and that it is quite possible (through extensive knowledge of relationship histories among young adults) to go beyond an intelligent evaluation of whether your boyfriend was serious about flirting with you or just trying to keep your heart's door open to him--since as I've indicated, it is clear that he had not yet closed his heart's door to you. And he probably has STILL not closed it with clear intention.
And that thought leads me to ONE suggestion I can give to you, so you don't just keep on waiting to see if his second romance crashes again, and then to see if he tries as hard or harder to get you back on board after that--possibly only to see him get involved again with somebody else new another 2-3 months after that.
1. DON'T be "friends" on Facebook, 'cuz it's not healthy for you to be obviously standing in the grandstands watching his new romance and rooting for him to strike out. AND it also reminds him consistently that YOU'RE still waiting for him: So he gets all the reassurance he needs that your door to love is still open, with him saying or doing anything at all. So why should he worry about being able to come back to you, no matter what he does? Why shouldn't he just keep trying to find somebody as good or better than you (in his own subjective perspective, not yours) and giving it his "old college try" with each one for the first 1-4 months to see if she'll measure up to whatever idealized image of you he might still have AFTER the rosecolored mist starts to evaporate from in front of his eyes and heart?
2. Write him an email and say "I don't want to be watching and waiting forever to see if and when you'll ever want to come back to me, even though I know you might want that. So I'm going to shut off contact on Facebook, since I also want to be free to do with my heart what I'm moved to do. But I ask this of you: Once you realize that you really want to go further with a new girl than you ever did with me, LET ME KNOW. Tell me you don't want me to wait for you anymore, but to find my own happiness with somebody new, if I haven't done that already. And even if you've broken up with a few other girlfriends, but you realize that you're past wanting to resume the love relationship we once had, LET ME KNOW THAT. The love we showed each other how to feel has been precious, so we each deserve to be blessed and supported by the other when it's definitely time to close the door to the past and concentrate all of our loving capacities on a new future partner."
I hope you see this and respond, so I can give you more understanding and advice if you want any.