It depends. If you have a free forum host, and you get to place ads on the forums, and you have lots of users with forums that have even more users + traffic, then yeah, they can be quite profitable. But it's tricky: unless you have the time + money to promote it correctly, I suggest you stay away. There are a couple of ad-free forum hosts out there, so you'd have to give your users a reason to register with you.
Very time consuming I can tell you that much. In order to operate a free hosting company, and get it where you want it ( profitable) will require a lot of time and a decent amount of cash.