I know a guy who runs a very successful forum...#1 forum for a particular car niche...has 110K registered users, and about 50K uniques a month, and 1 million page views.
Know how much he makes? $2,500 a month. $500 from Adsense, $1000 from selling car parts, and $1000 from direct advertising sales.
$2,500 is not bad, until you throw in the cost of servers and bandwidth....after all is said and done, at the end of the month he clears about $600.
Sure $600 is nothing to sniff at, but he's been running the forum for 10 years now....so you would think at that level it'd be making a ton more money than that.