Give up the fantasy she's such a wonderful child. She's a young adult lashing out at being tough loved.
Don't bother to read Facebook if you get your poor little feelings get hurt so easily there (and it seems you definitely do), eh? (they really are spying on everyone out of control so no loss) or if you just have to keep facebook, unfriend her and unfriend anyone who reposts her stuff.
It's a power game--if you act as though your world's ended and you're devastated beyond endurance, she wins and the abuse will just continue. She wants to punish you for daring to stand up to her.
Let's do a quick review--YOU are the parents, SHE is the child. My guess is you guys haven't got that relationship straight. Just avoid and distance her for now, she's in 'hurt mom and dad' mode, nothing's fixable as long as she is. Remind yourself you're trying to break some very unhealthy family dynamics here, part of which belongs in your lap, for letting this pattern get set up to begin with.
You surely know how counter dependent teens and young adults are; just tell them you don't want contact anymore and like any rebellious kid that's most likely what she'll want but keep her at bay until you see some real evidence of change. She may pretend so she can get close again to 'punish' you again.
And if she severs the relationship anyway, understand YOU can't 'fix' her, never could and lawsuits are just petty ways to avoid REAL confrontation. If need be, just let her go. I'm betting she'll be back if you can resist falling back into the 'ohhh poor meee I've loooost myyy babyy' box. Be an example of adult behavior.
Double masters in system counseling here. My further advice, get your self fixed instead of chasing this debacle around town and learn how be the adult you want her to be. That's bound to help far more than a law suit.