It seems that Meetup.com has a major UX issue, as it may not be easy for new users to figure that their service is meant to be used by groups that meet recurrently, and not by groups that may meet just once.
Having said that, I've been using Meetup.com for over four years now and I can only say but great things about the service. I used it to create the two largest meetups in Bogota: BogoTech http://www.meetup.com/bogotech/ and BogoDev http://www.bogodev.org/
I certainly see nothing on any of their pages to suggest that they ban more formal conferences. All their language is very broad and vague. The fact that "Organizer" is an official class of user who have to pay is a pretty strong indicator that it's not exclusively some loose informal thing.
Even in their terms of service, there's nothing that comes close to banning conferences. The only thing remotely related is that they disallow advertising commercial services.
Having said that, I've been using Meetup.com for over four years now and I can only say but great things about the service. I used it to create the two largest meetups in Bogota: BogoTech http://www.meetup.com/bogotech/ and BogoDev http://www.bogodev.org/