I don't have to wait to ask, since I pretty much know...
In my main group of 5 players, 3 of us have accounts on ENWorld. However one of them only has 20 or so posts and doesn't come to the boards anymore, he now refers to message boards as "stupid" (I'm thinking someone must have been an idiot to him). In fact, the other one has, when last I asked, over 20 people on their ignore list (Holy RPG.net Batman!) - I haven't seen them post here much lately. Not really a ringing endorsement...
Of the players no longer in the group, only one of them is on ENWorld. That includes 2 long time players and 3 that were only with the group for a few months. The one who is registered here is in the latter group, but I haven't seen many posts from him either.
In my other group, none of the other 4 players are registered here. Not sure about all the previous players, but I know at least 3 of them are NOT registered here (and I would be surprised if 3 of the other 4 were registered here).
I'm not as sure about other local gamers that I know. The only one I'm sure of was the owner of a (sadly) now defunct LGS, but again, I haven't seen many posts from him lately. I'm pretty sure at least one of the local WotC Delegates posts here, but I may be confusing his posts at Maxminis with here (broken record - IHSMPFHL). Considering I run events at local Cons (and used to run events a few LGS), that's not much.
So, anecdotally, WAY less than 10%...