Just to echo some of the previous replies, I use an in-game forum system as well. In a lot of cases, it's a little like re-inventing the wheel, because there are so many good existing forum systems out there. I took it as an opportunity to build a more generic messaging system, and so the same system that drives the forums drives the logging of battles and missions as well as allowing players to create posts on their own pages (sort of an in-game blog).
And really, when you're building an entire game, simple forums are pretty easy by comparison. Back about 12 years ago, when I first learned perl, it was so I could create a "guestbook". When I learned PHP a couple years after that, one of my first projects was creating a forum. A forum system has all the basic building blocks of any dynamic website with a community: user accounts, a relational database, different sorting methods, user-inputted text that needs to be sanitized, etc.