I say PC/Steam community will last the longest.
They have the ability to add mods and stuff in the future and thus can alter their gaming experience (which adds to the game's longevity--see
any PC game with mods like Elder Scrolls games, Sims games, etc).
Also, I think the PC/Steam communities tend to be more.... committed, I guess is the word or maybe have more patience? The console players will move onto other games quickly (at least, in my opinion), especially since there are always new FPS or multiplayer online games or AAA games coming out this year......
......At the same time, though, PC gamers may have more variety in games of various quality, so they may change to other games, too, lol. ^_^
However, Brink feels kind of a niche game, so who knows how the community will change, lol?
So....tough one to predict, actually.