Jordan
Ethnic, professional, sport, hobby, art, music... basically anything you can think of, in any combination, exists. Extracurricular activities are overwhelming.
There are frats and sororities, but they are not very prevalent in the social scene. The social scene changes completely when you transition from a freshman to an upperclassman. You move from the Yard to your House, where there is a lot more freedom. Moreover, you get to join "Final Clubs" which are Harvard's variety of Frats. The only difference is that they are Harvard specific, and people do not live in the Club Houses.
Andy
Extracurriculars at Harvard are a GREAT way to meet new people, get advice from upperclassmen, and build a lasting friendship. Small organizations foster a family bond through common interests or backgrounds. With hundreds of different student organizations, everyone can find a place where they can chat with friends of common interests.
Grace
Athletics aren't that big at Harvard, but some really good friendships and relationships are made on the athletic field, so it's definitely a good idea to join one. Crew is especially popular. Students don't generally leave their doors open - the administration actually frowns upon that for safety reasons. The best known extra-curricular activities are usually the big name ones: Harvard Radcliffe Orchestra, Hasty Puddings, etc.
People generally party once a week, I think, if they party at all. Spring and Winter formals are pretty big, and my social life on the weekends mainly consists of going out to eat, watching a show or concert, hanging out, and occasionally going to a party.
Harper
I meet my closest friends through either my entryway or activities that I do (which are mostly cultural/religious) There are loads of guest speakers who are pretty cool/famous, and it's pretty amazing to get to go see them. I find that my social life involves dance parties and shows, mostly because I don't drink, but there are many people who go out during the week (although they are not the majority) and get drunk. Frats and sororities are not as important on campus as they are on other campuses. You can definitely have a kickass social life and not be associated with any greek organizations or finals club.
Whitney
Most popular groups: The Crimson, the Lampoon, the Advocate, the Glee Club, the UC.
Andrea
Most people tend to socialize with people from their extracurriculars/dorms. Partying is less prevalent than at most schools, although there are always lots of fun things to do on weekends and parties are not limited to just finals clubs.
Lorie
Social life is as good as one makes it. There are no frats and most parties are smaller house parties. Almost everybody lives on campus which means it is easier to find your friends.
Jessie
It is busy, but students do it for credit and to better their Resume.
Bevan
Don't ask me, my social life sucks, that's why I'm in the computer lab at 11pm on a saturday night, filling out this form. I hoped joining the crew team (a very popular team on campus) would help me make friends, but it did the opposite because it gave me no time to simultaneously take hard classes and make friends. The crew, football, hockey, and lacross teams seem to provide big drinking parties on weekends. The theatre crowd is its own community and I don't know much about them. As for dating, a remarkably large number of students here are inexperienced virgins, which can be frustrating when you're just looking for a little fun. It seems like the only way to get laid is to get drunk at a party.
Christina
We do have a social life! We party, just chill, and do everything in between.