Damn...bad situation man. I know people are gonna say "don't care what people think blablabla" but this is like your entire friends group, especially if she posts it on facebook.
Why don't you talk to her and ask her why she's doing this? Like ask her what she's to gain? If she can't come up with an answer, that's half the battle over with!

I think the best way is just to talk to her. Don't come off as threatening in any way. This shit is like diffusing a bomb
Maybe make a new friends group? Or try to get closer to the one you have at the moment, so that they'll just laugh at you for a while and then forget about it because they love you so much
If, worse comes to worse, this stuff does get out, don't act all embarassed about it. A way to take your ex down completely is to deal with her how you would an AMOG. If she posts this, ask stuff like "woah are you stalking me?" and "ooh what were you doing on a dating site then?". Make sure you say this in a jokey way. Don't take the situation seriously.