Sometimes we have to live through the hurt caused. You can't 'tough it out' - she's your daughter; you're bound to feel pain with all this.
Totally agree with not doing anything to make matters worse. What's important is repairing the damage somehow, rather than being 'right', however unfair and unjust this indoubtedly is. As Eloethan says, your DD might have mental health problems, undiagnosed, which have caused her to react in this way. Big hugs coming your way - I know I'd be heartbroken if it was me. I sometimes find Facebook a mixed blessing, for various reasons, and this is a case in point. There was a time when my DD didn't like our response to something she'd told us and rather than keep this private and in the family, she chose to broadcast it over Facebook, rather hurtfully. In the end I emailed her (we were away on holiday at the time) and as gently as I could, said that there would always be times when we didn't agree, for one reason or another, but that we'd been hurt by the fact that what we thought was a private family matter was now subject to the judgement of other Facebook users. I didn't rant (though I wanted to!) and finished by saying that we loved her no matter what.
She deleted the Facebook posting without further ado, to our relief.
I hope your daughter will do likewise - it's more important to sort differences out within the family. Thinking of you, Thistledo!