This really has been said so many times before, but you get out of Oxbrdge what you put in. If you want to stay in your room and study the whole time (and I know of people who do!) then that's OK- in the same vein, if you want to go out every night and get bladdered then no one will care- apart from your tutor and DoS! As you would expect, most people are somewhere in the middle, maybe going out after handing in an essay to celebrate, or on someone's birthday, or whatever.
The college bar is perhaps the hub of going-out kind of social life in college. People tend to gather there, and then move out to clubs and so on, or just stay around until last orders then go to someone's room and just chill. In fact, I'm not really into going out clubbing, because they are too packed, too loud and too expensive, so I prefer to just meet people and relax.
Like I say, there are extremes of attitudes here. If you want to go out every night, then I can guarentee that there will be people for you to go with. If you want to spend every evening in the library until midnight, then you won't be alone in that either.