Forum, or BBS, looks a bit outdated when most people are talking about web2.0. But this is a wrong perception.
Forum is still one of the most efficient ways for discussion. When the community is active and large, the manageable discussion can be easily achieved through forums. If you use social network, the discussion is unmanageable, and the discussion is fragmented. This is the reason why most active communities have one or more forums. For example, if you look at Google, all their support is through forums. Can you image they can do it through Google+, or facebook, or blogs? The answer is no.
One of the major demands for free forum hosting comes from those loosely-organized online groups, for example, game buddies. They need a place to discuss or exchange techniques for certain games. Forum provides threaded and organized discussions, which is the most efficient way in result-oriented communications. In other words, people knows what they want when they go to a forum. On the contrary, on a social network, most people just hang out there mindlessly. Social network simply increases the number of such groups, and therefore increases the demand for free forum hosting.
Free forum hosting is not as easy as free WordPress hosting. As explained in another article, you simply enable network to enable free WordPress blog hosting. For forums, up to now, there are no comparable scripts for forums, and you need some third party scirpts. But don’t be frightened. Free forum hosting, at least free phpBB forum hosting, is not as difficult as free Joomla hosting. In the market, such scripts have been existed for many years. Only one installation of phpBB is needed, just like WordPress.
For phpBB2, there have been several scripts around. The most widely used is sebflipper’s (Sebastian Flippence) Multi Forums Pro Host. But it seems he abandoned this project, and there are no updates for phpBB3.
If you are looking for phpBB3 hosting script, you have more choices. There a few free ones (search phpbb.com, and phpbbhack.com) and a few customized ones. The most commonly used (not free) is phpBBHS, which is easy to use and has most features you want.
Anyway, the demand for free forum hosting is increasing, the market is not saturated, and the initial investment is not high. So, it is a good opportunity for free hosting providers.