The biggest other site as far as I know is "Aspie Central". Supposed to be pretty good.
That's about it. I checked it out once. Looked like a good lively site.
I used to be a regular at "the Aspie Hangout" on Delphi forums before I discovered WP. There were technical problems with the hangout for a while and I couldn't visit it. Its up and running now, but when I visit it now its twice as dead as cemetery. Just a sad shell of its former self.
A lady who was a member of WP got angry at WP, and motivation by little more than revenge started her own rival site called "Right Planet". But even she doesn't visit her own site anymore, and neither does anyone else. I am member myself, but there is never anyone there. It also suffers from having the same name as a far right political site.
So there are some ghost town sites. But not many big active sites.