I don't want to talk about specifics, but I was involved in a forum a while back that was founded by someone with no prior knowledge whatsoever on a free forum host. It's no longer something I'm involved in as I've moved on personally, but it's now at 30,000 posts and is an active small community with its own hosting and a domain it did not obtain until last year. Not all forums are quite so lucky, but I don't think free hosts kill the chances. A lot of people don't really care and just want a friendly community to be a part of, so in a lot of ways, it's really just about finding the right niche and the right places to promote. For a lot of forums, not having a domain can hurt though.