I think bitcoinsforums.net in particular is becoming more and more well known (I keep seeing it mentioned in other threads here), but there is little incentive for a person to change to a new forum without many other people changing to the new forum as well.  If this forum were to go down completely, for a day or longer, people would find these other forums rather quickly.

But, if you wish to help decentralize, come post on that forum as well.  Every post has the potential to make it more attractive to more people to encourage more people to join.  We just haven't reached that critical mass yet.