My boyfriend is a PhD student in electrical engineering, and most of my close friends at Cal are also grad students. I'm an undergrad. How did this happen? Well, I live in International House and I met them all here. I would strongly advise living here or a dorm specifically for grad students if you want to meet people. I-House does not let lower division undergrads live here, so it's all juniors and seniors, grad students, post docs and visiting scholars.
It's really easy to have a social life at I-House. I wouldn't consider myself outgoing at all, but you just meet people by default here. There's a mandatory meal plan, so you all eat together. You'll probably become good friends with the people who live on your floor, and when the time comes, a bunch of you will probably want to move out together (there are three or four of us who will be moving out together in the fall, after more than a year of living here).
There were at least 6-7 engineering grad students on my floor this semester, and most of them would spend all day in classes or labs, but came home at the end of the day and chilled with people, went to a bar, dinner, party or whatever. Comparatively, my boyfriend's research partner lives alone in an apartment and does not have much of a social life. I think he wants to possibly move out with us next semester.
Last things -
Pro - International House is made up of residents from all over the world, so you'll get a culturally rich experience living here. I'm not sure if I quoted that from the mission statement. It's possible.
Con - The food sucks. That's mostly why we're moving out. But if you can tolerate it until you have enough of a social life, it's totally worth it.