I suppose you could count the number of hits the website gets in a month? But again, I don't know what 'factual' data that tells ya.

Same problem; a significant portion of those hits could be from bots.

If the admins have some sort of method of sorting the users by last activity, I really don't see any difficulty in counting the number of active users.  Just search for the ones whose latest activity was in the last month and have more than say, about 3 posts (Most bots would probably have 0 or 1) and see how many users that returns.

Or hell, it probably wouldn't take too long to count by hand.

Tada, look through this site's membership. Anyone signed in is 95% guaranteed NOT to be a spammer. I check new member profiles every week too, so we have zero active spammers here.

I just had a bit of a hard time believing that with 10+ people on at any given time, the boards are as slow as they are.

To further clarify...
Users: Registered & Signed in
Guests: Google search visitors, members not-signed in, & internet crawlers.
Total: Combination of those both.

So only the "Users" count are the ones who can post.

I've also done a good job of keeping the spambots out. I think we've only had about a dozen or so registered at one time but with the registration passphrase enabled, that's gone down to zero.

Pretty much all but a few of our users still keep in contact with us on Facebook. I can count 1 person who doesn't have a facebook account because they don't like social networking sites...

To reiterate:

I find it very hard to believe that there are 15-ish people on our site at any given time.  That would be like, every regular member here signed on 24/7 and a bit of random internet traffic on top of that.

"Random internet traffic" makes up 57% of the traffic to the site.
This is the traffic from the previous 7 days to the site.

There are a lot of old communities that are still half-way online, but dead. I figured I'd link them for nostalgia sake.
http://www.members.tripod.com/holy_knight_7/home.html
http://members.multimania.co.uk/ddrba55/
http://www.brandon2u.8m.com/news.html
http://ddrtacoma.tripod.com/
http://imperialpints.tripod.com/index.html

It was awesome when there were dozens of players at every DDR location you could imagine. Those were certainly good times.

Btw, I like how this guy rates machines on his page.
http://www.brandon2u.8m.com/news.html

He has a category called "Biggest Drawback" and it mentions things like kids, crowds, whatever. A nice idea. It'd be cool if more rating places had Pro/Con sections...though I guess letting people leave comments on those locations probably satisfies this need adequately.