If you're just looking for people, wouldn't Facebook work? I would think that self-hosting is preferable in that things can better reflect the character of you

If you're just looking for people, wouldn't Facebook work? I would think that self-hosting is preferable in that things can better reflect the character of your meetup instead of everything being generic (I never liked Meetup.com for that reason). Also, it's good to avoid lock-in. So what about an aggregator for everyone's meetups? Maybe an API somewhere that you hit with your meetup info and that's a place to find the audience, but the aggregator tries to push them to your site?