I have the same problem, but lately it hasn't been for lack of trying with the few people I know. Some of them just don't bother to call me back, although they make a big deal about wanting to meet up and do something, which I guess is just polite BS. The other friend will do something with me if I call her, but she hasn't made contact with me in the past 7 months. So I guess they have better things to do.
It does get severely depressing after a while. I've been told I just have to keep trying, and I guess that's true. Part of my problem is that I meet a few people, we don't really click, so I think I'm a hopeless case. I guess I have to try and talk with more people, which I don't exactly have an easy time doing, or a high tolerance for. Maybe some people are just not as appealing to the general population, and so it's harder for them (me) to meet like-minded individuals.
Hopefully eventually I'll meet others who actually enjoy my company and give me a call once in a while rather than me feeling like I have to make all the effort or I'll never hear from them again.