I just found this site that aggregates data about the size of internet forums. Naturally, I had to hunt down "my" site on there!
Turns out good ol' TFP is number 458 out of the 1642 that they know about. They've got some neat graphs of post count and member count over time, too.
See
http://www.big-boards.com/highlight/729/
458 is pretty good, but I'll bet we can do better. C'mon, folks! Put your back into it!