How do people make money on websites where nothing is sold? They just share advice, etc.?

I've always wondered that, like craigslist, or gossip sites, or where people just talk, how do the people who start the sight get rich?How do people make money on websites where nothing is sold? They just share advice, etc.?
They will sell advertising space.How do people make money on websites where nothing is sold? They just share advice, etc.?
its all about the adds....more interesting stuff = more traffic....more traffic = more money from adds....its kinda a job...u work hard 2 get traffic....i wish i could start 1 :D
something HAS to be sold. it's advertisement. advertisements bring potential buyers. for example, i will get paid for putting the following link down:





http://x.vu/555





Someone will go there, become my referral, and I will get paid for that because some of those referrals are bound to buy something from who I am referring them to.