I've used forums several different ways in roleplaying games and I've found it to be a deeply rewarding experience.
I once created a forum with about a dozen different accounts that were all mine which was a bit of a prop / plot hook generator that was called Paradelaide and was focused on several paranormal investigation groups in Adelaide. When the vampires in the then Camarilla game stumbled across it, they got to register an account and either investigate it, encourage it, or put up misinformation, as they so chose. It allowed the chance to select interesting ghouls as well as help them find out about masquerade breaches in a more organic and interesting way.
Forums can also be used to add some between-session consistency in a game that has frequent downtime periods. I created one for my Vampire: the Requiem LARP game with a News section so I could let them know what was going on; a section for the Court to disseminate information / misinformation and chat to each other; a section for each Covenant to have a chance to get their group politics on and make plans for the future; a section for each Clan to do much the same (was less well-used) and a Your Forum for each player so that they could do their downtimes, chat to NPCs, and follow Personal Plots. It worked out brilliantly well and had a great uptake. The game only folded because of the general LARP attrition rate competed with the low recruitment rate of a personal game.
Then we have a forum for the Demon: the Fallen game which is divided into Court, Faction, and Your Forum. It's less well-used than the Vampire one but that's partially because there's fewer players and partially because, well, it's not a political game. It's an episodic investigative game and thus most of the meat is during session time. Really, the forums here are more about keeping everyone up-to-date on the occult information found in downtimes or chewed over between sessions. Adam and I are working on making the Faction sections more interesting by using multiple NPC accounts and we'll see how that goes. It may encourage more globe-trotting and will certainly give the players a greater chance to pick and choose their leads, but I'm pretty enthusiastic about it.
So there you have it. Some information on using forums in such a way. If anyone's interested, I can link you to the old Vampire forum so you can take a look around. Just leave your email address in the Comments box and I'll send you the link and Register you a Take-a-Look account.