I make (well, past-tense at the moment) almost $2k/month on a vBulletin play-by-post gaming forum I started back in 2007. One single adsense banner below the navbar.
I actually burned out within 6 months of trying to bootstrap the fledgling forum with fake activity, clever backlinks, and entertaining the trickle of registrants.
Burned out enough to take a a break for a while. Came back months later and it was a bustling forum of activity. Apparently I'd just reached that critical mass necessary for the community to be autonomous (able to entertain itself and cajole newbies to stay) before I took that break. Nowadays, I do very little beyond pay the server bill. It's staggering the amount of work volunteers (moderators) will put into maintaining a community and I'm grateful.
Recently had adsense disabled on my website after some automated process decided my website was "mature/adult-themed". The automated email cited a post in our forum's off-topic spammy section where some user copy and pasted the phrase "sexual intercourse" over and over again in Mandarin. Just some non-Mandarin-speaking teenagers being silly for a moment. Now I'm working on getting adsense reactivated.