As far as freeware game communities go, isn't DF pretty huge? This seems like a pretty active forum.
It is some what huge. We have been referenced in World of Warcraft and Super Scribblenaughts, so safe so say DF has established it's place in the gaming world. Despite this, it has a similar position to the works of Lewis Carroll, in that the amount of people who know about the works, and have even dabbled a little, far outweigh the amount of people who are familiar enough with the game to feel comfortable to click the link to the forums.
While this is a very active forum, it is still limited in it's number of users. For example, I know to some level most of the people on this thread, and I'm sure at least one or two of you know me. Other forums out there work more like a public shout box, where relationships do not extend beyond the scope of the thread.
So, for example, I know Aqizzar is a highly intelligent and respected user, I am somewhat sure that Realmfighter is insane, and I know how MoM gets so many freaking posts.
Edit:
Yeah, I would say we are pretty huge. Right now, certainly not a peak time, we have over three hundred people on. I would not be surprised if say... A thousand were on at peak times.
2228 was the most we ever hit, and as I recall that was the day we upgraded from d40 to what was called d2010.