Forums across all boards have been experiencing slumps in the main member base. 
I've seen it talked about at 2 unrelated forums to here, they were talking about how a lot of old members no longer post or gone. Some of them have been around since 1998, I have been a member there since 2002, 2003, etc.
Some reasons why there could be less deaf people might be the same reason the think they have less people on those forums:
1 People stopped going on the internet as much
2 Facebook started around 2005 or something, but it was quiet until it boomed in 2008
3 Twitter / myspace
4 Youtube / vimeo / deaftv for us here
2, 3, 4 are the big theory for a lot of sites why forums are losing members. A lot of new members joining today are just asking questions about something that usually doesn't get answered well on FB or other places.
