I really like this discussion. It's something I haven't thought much about previously.
I feel TMK provides its information at a much better quality than sites like MarioWiki. That is, it's clear, concise, and enjoyable to read. Although the quality of pages on MarioWiki aren't up to par with TMK, there's a lot more information available about the Mario series as a whole. On top of that, anyone can easily contribute to pages and share their knowledge.
In general, I think social media sites like Facebook, Twitter, and whatever else is out there have for the most part dominated online forums. For example, SmashBoards used to be the premier site for organizing Smash tournaments, but now people have moved away from it and shifted to Facebook Groups/Events. Convenience may be the biggest factor, considering Facebook is many people's central hub for social activity online.
I don't post here very often anymore because I too am a game designer/programmer and prefer to spend my free time working on personal projects. I still browse the forums every now and then, though.