I've seen it work for nerdy singles and some outdoor activity groups. But other than that, the numbers just aren't there. People sign up for groups and never go. Look around and see how many groups with 500+ and even 1000+ people who only have 1 or 2 people showing up to scheduled events.
People don't use the Meetup interface after they've joined. I've seen it myself, and heard this from several people who ran groups and then closed them down.
I took over as organizer of a group from someone who'd started it and decided to leave. I kept it going for almost a year, and even grew it a bit. But the bottom line is, it did not bring people to our meetings.
Use Facebook or set up a website if you want to get something going. Meetup is lame.