The money comes out of Donations to the forum, from ages ago.
The donate button on your frontpage beside search.
Actually your right it is from ad revenues, the forum software was from the donation pool.
Had those two mixed up there thanks for the correction mexxer it's a page I haven't used in a long time so I didn't notice the Theymos note till you brought it up subconsciously
.Appended here:
The forum sells ad space in the area beneath the first post of every topic page. About 25% of ad income goes to the forum moderators as thanks for all of their work. (There are many moderators, so each moderator gets only a small amount -- moderators should be seen as volunteers, not employees.) The rest is stored in the forum's treasury (verifiably), where it sits until the forum needs it.