Various members of fandom are attempting to get a headcount on how many journals of the 13 Mil on LJ belong to members of fandom. Join fandom_counts with all your journals: personal, writing, RPG etc to be counted.
More about the effort here.
Also, if you moderate any fannish communities, modify the user info with the interest fandomcommscount, so that by searching interests using that as a keyword, the number of fannish communities that exist can be established.
If you know anyone who doesn't know, pass the word on.