I'm pretty social and would love to meet more rationalist friends, but I have the perception that if I went to a meetup most people would be less extroverted than me, and it might not be much fun for me.

My experience at meetups has been pretty social. After all, meetups select for people outgoing enough to go out of the house in the first place. I'd encourage you to go once, if there's a convenient meetup around. The value of information is high; if the meetup sucks, that costs one afternoon, but if it's good, you gain a new group of friends.