Dagnabbit Tibi, you just stole all the best stuff I was going to say.
Bridget - how big is your chapter and the school/town? The bigger a chapter is and the more social options there are, the more cliques there will be. That's just a fact. If there is only one place to hang out at in the whole town obviously everyone will end up there.
I'm going to play devil's advocate here. You say you really don't like going to clubs and such. Is it possible that you (and some of the other sisters who are feeling "left out") expressed that opinion and so the girls who were going clubbing thought "if she hates it so we won't ask her to go"? I mean there are sisters of mine who hate that type of thing too - so I'm not going to ask them. But I really like doing it, so why should I stop?
We always had things like the whole chapter going to Perkins (a Denny's type place) after rituals and big-little. Try to get something like that started, rather than trying to "break up" people who have formed bonds.