I've never made friend's easily, and so don't have many people that I'd call such. Most of the people I've met are acquaintances--I know their faces, maybe their names (I'm horrid with names), and I can engage in small talk with them, but I wouldn't tell them anything I wouldn't mind the whole planet knowing. I've got less than ten people I'd actually call friends, and three best friends that I can tell anything to and feel safe.
To qualify as a friend, the person has to earn my trust over time and prove themselves loyal and honest. I can't stand wishy-washy people. I want someone I can count on when I'm in trouble. In the same manner, I'll return the favour. However, slight me, and you'll lose my trust, which is something very difficult to redeem, though it's easier if said friend is repentant.
This doesn't mean I'm not social...I'm just very insecure and tend to be almost paranoid of people in general. It's easier to be friendly over the Internet, I guess, because I don't have someone's eyes on me, making me skittish.