I think it is the witnesses who vote on the fork. If you look carefully on your witness page, there is a fork version vote and fork time vote. I suspect that the fork happens automatically when most of the witnesses are running on the updated software and the agreed on fork time arrives. Someone, however, must be in charge of switching over the steemit website to the right chain. That, most likely, could be a point of central authority, even if the block chain does not have one. On the other hand, I suppose it wouldn't be too hard to put a new website up, Steemit Classic, perhaps?