I agree - some just love talking about their passions. Also, it is a known technique not to monetize at first...
If those ads are sponsored and not PPC, then that is your answer. With a site that has that many active members, they are charging a mint for advertising fees. Just look at domaining.com's advertising fees, it makes you want to puke:
He (equinoxprime, owner of the Purse Forum) spells it all out here (well more or less) in another Thread: "How's this for SE & word-of-mouth traffic"