Do you mean something such as
http://forumup.com
To be honest I believe they don't do very well. Before I started getting into hosting properly, and websites, I used them and they place millions of adverts of the website to cover the cost.
Lots of forums = lots of CPU.
Lots of CPU = pay a lot out.
Pay a lot out = you need money in.
Getting money in = Advertisters.
No advertisers =
YOU SHUT DOWN
Then you have wasted around $500 unless you can open a website worthwile.
Remember that everything is along a chain. I'll carry it on.
Lots of adverts = people leave
People leave = No advertising
No advertising =
YOU SHUT DOWN
VERY VERY hard to run a site like that, unless you are a massive company, someone that can afford it, or you get popular straight away.
Thats why I do know of only 1, (forumup.com), and have ONLY EVER heard of 2.
So few for that reason.
Take my advice
CJ