This has become a huge problem for me over the last several years. I went to clubs a lot as a teenager as an escape from problems at home and school, as it was comparatively comforting to be around other "weird" people. I was socially awkward and didn't dance, but wound up dj'ing for awhile because I found that I could really immerse myself in music. I even made some friends, most of whom were a bit older and had a sort of protective nature that I found comforting. When I hit my 20's and stopped being involved in all that, the phone calls slowed to a crawl and the few parties I went to left me feeling uncomfortable or bored. Everyone always seemed so slick and sociable, while I was usually the guy that wound up nursing a drink by himself, chain smoking, and occasionally, someone I knew would drift by for a couple words and a pat on the head. When the parties got more ostentatious, with people dressing in elaborate costumes or expected to bring things, I gave up and just stopped going.
Since then, the phone calls have slowed to one or two a month, I'm not invited to things, and the rare occasion someone comes over, I get the impression that they're bored and want to leave. I'd call or IM people more often, but don't really know what to say, and nobody really tries to reach me anymore, so I just figure that they lost interest.