Whitman is a pretty social campus. Like I said earlier, there is always a lot going on. On any given Friday you can go to Coffeehouse on the bottom floor of the campus center building (complete with a stage and comfy couches) and listen to local or student bands (and the occasional slam poet) play for a few hours. There's usually great music, and the cafe upstairs is open so you can get some coffee or tea too.
Whitman does have a Greek system; there are three sororities (Kappa Kappa Gamma, Kappa Alpha Theta, Delta Gama), and four fraternities (Sigma Chi, Tau Kappa Epsilon, Beta Theta Pi, Phi Delta Theta). The fraternities have houses on the north side of campus, and the sororities are housed in the all-womens' dorm. This being said, you don't have to join a fraternity or a sorority in order to have a social life at Whitman. I personally chose not to join a sorority, but several of my friends did and they really enjoy it. And the great thing is that I can still hang out with them and their sorority friends. The Greek system is not exclusive or in-your-face crazy, as it might be at other schools, which I find really nice.
As most other colleges, there is a party scene on campus, but like the on-campus Greek-life, it's not in your face all the time. If you are looking for a party on any given weekend, you will probably find one. However, you DO have to look for them; they're not going to find you. In my opinion, it's nice not having a raging party scene all the time because then if you don't want to be part of it, it's easy to not be.
That being said, one of Whitman's more amusing traditions is Beer Mile, which is sort of what it sounds like. At midnight before reading day of second semester finals, students gather on the main field and run around. Naked. It might sound weird (or totally cool), but I guarantee, it's one of the more fun/funnier events that takes place.
Dorm life is obviously a huge factor when it comes to being social. For the most part, students keep their doors open and often time choose to hang out in the lounges rather than in their rooms all the time. The RAs of the houses and residence hall sections occasionally put on events where people can just hang out. In my dorm during my first year, we had several open mic nights where everyone would gather in the lounge, drink hot chocolate and eat snacks, and listen to their hall-mates serenade them with their lovely music.