AFF has gone through some troubles recently and has a tendency to go down every so often. Which is regretttable, as it is a truly aspie/autistic site and I hope it stays around for a long time. But, when a site becomes "unreliable" for whatever reason. People will bolt to other sites. AspergianIsland is a major case in point. It used to be busy and productive, but when it went down a couple of years ago and there was a lot of acrimonious accusations of thisnthat. Many members left and never returned.
As for member count, I can't take that very seriously. Wp member count probably includes a lot of spambots, sockpuppets and banned members. And I can tell you for a fact that a lot of AI membership is spambots (mostly) neutralized and banned members and sockpuppets. If I had the time and patience I'd be getting rid of those, as I just don't feel good about keeping these superficial/artificial member counts as they are.