...after 2 years at Haas, it's kinda fun to revisit this topic. I figured I'll share my experience. This is in no way an ideal way of doing things, and I'm not here to tell you what to do - just telling you what I did and where it got me
Basically, it's exactly the way everyone describes here. If you want to network, you want to socialize. If you want to socialize, you want to attend things. Soft club memberships (beer club, wine club, whatever club) are VERY essential: people get drunk, embarrass themselves and build this "we puked together" connection.
Now, my perspective on this was a bit different
I do not drink, I didn't want to network, and didn't care much about socializing. I didn't sign up for all the social stuff. (Well technically I did, but never attended.) As I mentioned here 2 years ago, I came in for a bumper sticker and for knowledge; I got both.
So did it impair my networking? Oh yes, big time. I'm a huge outlier on the "class relationship map", cause I have just a few friends and a few study buddies.
Did it hurt my job search? Not at all. After-MBA job search proved to be completely network-independent, at least for Haas. (By "network" I mean classmates, not employees of the companies you're applying to. Those guys you DO have to court, on those events you DO have to show up.)
Will it hurt my future career? Maybe. As truly noted above, who knows where my classmates are in 10 years, and whether they will hire me if they hardly remember who I am.
Do I regret not getting social at Haas? Yes and no. Yes, because it's just a useful skill to have (being able to communicate nicely), and it's not something easy to get, especially if you're a foreigner and English is not your first. No, because I'm a "ronin" type of guy who wants to achieve everything by himself. I don't expect my buddies to hire me; I expect to move fast enough to hire my buddies
I might not get to the very top this way, but that's the kind of game I play. As one of my friends noted (see, I do have some!), "you want to win the game on YOUR terms". Exactly so.
P.S. This post probably paints me as an arrogant egocentric ass that doesn't care about people. Not at all
I'm super-friendly and built awesome relationships with everyone I interacted with at Haas. It's just I didn't actively go out and hunt for those contacts, letting them happen in a "natural" way.