Hi, and thank you for asking your question.
Safari's Website Data keeps track of each and every single item, including buttons, cookies, etc., that's present on the websites that you visit. For example, if you visit a webpage that has a Facebook share/like button on it, Safari will store data for that specific button as well. Judging from the tiny amount of data shown for Facebook, this is the reason for it showing up - if the user were to actually open up and log in to Facebook, it will use much more data than what's being shown on your device.
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Hmm, I see. I just tested using my iPad and it used up ~100KB of data just to log in to Facebook, so it looks like the data used might have something to do with the individual device's network connection as well. If you are making sure to clear out the website data each time you use Facebook, and if it still shows up there anytime after that - it definitely has something to do with either the Facebook share/like links on a webpage or a third-party website that has some connection with Facebook, e.g. if you use Facebook to log in to any other site like Instagram. Something else I can think of is, Facebook regularly serves tracking cookies across the web to track users' behavior and provide them with relevant ads based on this activity, so this might be using the minimal data you are seeing, as well.
It's practically impossible for anyone to remotely use Safari on your iPad to access specific websites, if you are not accessing them yourself. Please accept my apologies if you are still not clear about this, and if so, do let me know and I'll open up your question to other experts who can hopefully shed further light on this if needed. Thanks!