Update: OK, I can be a little lazy, but the answer was pretty easy to find. From their "about meetup.com" page:
How will you make money?
We plan on two main ways: (1) We'll get paid by retailers that have good MEETUP spots (e.g. cafés, bookstores, big pet stores, restaurants, videogame retailers) to "slot them in" to Venue Voting. Like any business, these companies will pay to increase traffic and sales. (Note: We never force a MEETUP to be at a particular venue. MEETUPs only happen at places that the people vote for.) (2) We'll get paid by users for access to some upcoming, extra cool features.