A couple of times a week, pro'lly starting when he was about a yr, now it's slowed down b/c of new baby bro--for a while maybe 3-4x a week. For a while I had all this guilt--I SHOULD be exposing him to other kids, he's missing out, he needs to socialize!! blah blah blah. But you know, he just turned 3 and he's just now expressed a real desire and enjoyment to play w/ other kids--before he'd just watch or parallel play. So I wouldn't worry so much that you don't go out a lot,b/c it isn't like they really even play w/ each other until 3-4, y/k? I think the benefits for us prior to 2.5-3 were getting out to do something different and me seeing my mommy friends.
I found my awesome playgroup thru LLL, but they're a homeschooling group and I could have found them thru Yahoo if I had looked. I'm also in a Meetup group--did you sign up for notifications to things you're intersted in? I get notifications whenever a new SAHM/AP/preschool playgroup meetup starts. My Meetup group does a lot but it's one mom who organizes it all--volunteers her house for playdates, fire station tours, meeting at the mall or whatever--maybe you could do that? I'm too lazy, lol. Yahoo groups is good too.
We moved here from another state 5 yrs ago and I'm just now feeling like I have a pretty good social network, and I think I've been pretty proactive and have worked at it! But anyway, I think your child pro'lly enjoys being around you more than any other kids at this point, other kids/people are just an interesting extra.