I guess it depends on your interests, but I'd suggest going to meetups centered around activities that necessitate or strongly encourage socializing. I go to meetups to speak a different language, and I tend to make pretty good friends at those since the meetups typically revolve around just getting a meal and talking with everybody about life.

Aside from that, maybe you could try taking up a hobby you didn't think you'd be interested in? Pretty much any reasonably sized city seems to have meetups for learning how to salsa dance, for example, which is inherently a very social scene where people all get to know each other.