It is interesting for me to watch my kids go through this. What I have concluded is that sororities/fraternities play a much larger role at large schools than small schools.

My college (Brandeis, a medium sized liberal northeast school) had no fraternities/sororities/football team, etc. The rebel in me liked that. I got a job in a research lab as freshmen, so for me my family/social group were my fellow labmates.

My son attends a large northeast school (MIT). His experience is similar to Deb's. The school is so large, fraternities provide smaller communities within this massive intensive place. They give them a sense of family, a house to live in with a cook that makes them family style meals that they eat as a group, etc. While his fraternity is not co-ed, there is no shortage of women friends that the brothers interact with socially. My son's brothers are his friends, and I imagine they will stay friends for a very long time. As Deb said MIT is a very intense place academically, and the brothers help each other with their work. My son said he learns best by teaching, so really enjoys tutoring his classmates. So, while of course the fraternity has a social aspect to it, to me it appears to have more of a community/family/study group atmosphere.

My daugher attends a small northeaset women's liberal arts college (Wellesley). I don't think they have sororities, although they do have some sort of groups that are similar. My daughter doesn't belong to one of these, but did get involved early on in a cooperative that runs a campus cafe, and the friends she made through that became her social group. The dorms are more house like than at a school like at MIT. Each dorm has a living room, dining room, kitchens, etc. and it feels more like a frat house than a dorm, perhaps another reason sorotities aren't necessary there.