Dartmouth social life can be summed up in one word: pong. For those of you who think pong is throwing balls into cups filled 1/6 of the way with beer, you are mistaken. For anyone who went to Dartmouth, you know that pong involves ping-pong paddles with the handles cut off which you and your partner use to alternately volley the ball back and forth across the table, attempting to hit or sink the ball into your opponents' cups, forcing them to drink warmish keystone light. Pong really is almost a culture at Dartmouth, as most Greek houses have multiple pong tables in their basements, and any Wednesday, Thursday, Friday, or Saturday night you can almost guarantee that all of them will have lines 3 or 4 deep. Asking a guy or girl to be your pong partner in next weekend's tournament is akin to asking them out on a date, with a possible hookup at the end of the night.
As you probably have guessed by now, fraternities and sororities have a large presence on campus. Since the town is so small with few places to go out and the surrounding area doesn't offer much either, much of social life takes place on campus, which often means Greek houses. People who aren't in Greek houses however won't have a problem hanging out at them and being friends with their members. There were plenty of people who weren't in a frat or sorority, but hung out either at a particular house all the time or at various houses. I also believe that fraternities and sororities, especially the local sororities, are much different than frats/sororities elsewhere or your traditional Greek houses. At another school (especially a southern one), I probably wouldn't have pledged a house, but there's none of the "we're so pretty and perfect" that I think of when I think of traditional sorority. All the girls at theta where I pledged were so friendly, welcoming, and accepting. We had everything from lesbian rugby players to bookworms to blonde track stars to stoners. We may not have been the prettiest or "coolest" house, but whatever, we had fun and I think were generally known as friendly and accomodating. My sorority was definitely where I made most of my closest friendships and had the biggest (almost all positive) impact on my college career.
There are definitely things to do on weekends if you don't like greek life or drinking, but I'm not the one to ask about that. The college sponsored a lot of non-alcoholic late night events, but any that I attended were mainly drunk frat kids looking to score some free pizza. There are some great restaurants, cute shops, and quaint movie theater in Hanover, but for real social life the majority of my experience is Greek houses, with a some dorm/house parties thrown in. Sports teams form another type of major social group, but since they don't have a house or central place, they tend to either mostly all pledge the same house(s), or hang out as a group separately.
The best social events at the college are the big weekends each term: homecoming in the fall, winter carnival in the winter, green key in the spring, and tubestock (now fieldstock) in the summer. These are all 4-day long weekends usually starting Wednesday night, where you drink way more than you ever should, try to make it to some of the non-drinking events (speakers, ski races, dinners), but usually wake up monday morning realizing you've spent most of the weekend drinking in a frat basement, except for when you were drinking at the bonfire on green or at the block party at Phi Delt. But these weekends do instill a good sense of Dartmouth pride and make you resolve to do all those non-drinking events when you come back as alumni.