I think it's a pretty common problem--or at least, it seems that way to me because people are pretty understanding. If I say, "I'd love to hang out, can we make plans right now? I've found that if I don't, then I just say 'yeah, someday' and it never happens," people usually make plans with me right away. Sometimes I have to kind of force myself to take that first step though, because my introversion loves to talk me out of social engagements, but I know that once I make myself go, I'll have fun.