I'm Lucy, and I'd be happy to answer your questions today. I'm sorry to hear that this is happening.
Threatening to beat someone up is a crime. These girls may think it's funny and they may be all talk - but they are legally committing stalking, and they could go to jail. Cal. Pen. Code, Section 646.9. Your daughter actually could file a police report for each and every event, and since this is all social media - which is so incredibly stupid - your daughter has all the evidence. All she has to do is hand it over to the police. They could conceivably file one misdemeanor charge against each of the girls for each threat or act of harassment, so, depending on how long this has been going on, that could be multiple charges.
The other option, if you haven't already, is to take printouts of these threats and conversations and sit down with the girls' parents (either set or both, if possible). Request a meeting, show them what their children are doing, and see if they'll take action. California law allows an injured party to sue a parent for a child's willful misconduct, which includes personal injury, so if your daughter is harmed, they are liable. Cal. Civ. Code, Section 1714.1. Hopefully, letting the parents know that will get their attention, even if doing the right thing isn't even incentive for them to act on their own.
On top of all that, your daughter could sue the girls who are harassing her (and their parents) for intentional infliction of emotional distress if you have medical bills, therapy bills, etc. showing how this has affected her. Whatever you decide to do from a legal standpoint, I hope you are able to get your daughter the help she needs. No one should have to deal with what these girls are putting her through.