Social apps, mostly games, on Facebook and MySpace can make a lot of money. As long as you have a good game play designed, you have a couple of monetization options that are commonly used/proven by the existing players today.
Players of social games are ready to pay for a) customization and decoration of the game environment; b) advancement in game levels. For example, Tencent made a lot of money with premium instant messaging, pet penguins, page decorations. Playdom did with clothing, weapons & adornment for cars, and advancement in the game. Zynga did with poker chips, mystery crates, home improvements, flame throwers. (Via: paidcontent.org)
Free v.s. Pro
Games designed with two or multiple levels of players can charge for a Pro experience. For example, in Playfish games “Who Has The Biggest Brain?” and “Word Challenge?” users have the opportunty to access additional content by “Going Pro” payable either via payment methods like PayPal, PayMo or PayByCash, or by using affiliate programs such as Trial Pay.
Social games can also monetizing their audience by advertising, with direct or through a ad network. For direct advertising to work, you probably will have to already have a sizable player audience so that direct advertisers are willing to work with you.
For advertising through an ad network, you have quite a few options: Offerpal Media, adparlor.com, tattomedia.com, Super Rewards, etc.
If you are still looking for evidence on whether there is a serious business in social applications and social gaming, just take a look at the big players such as Zynga, Playfish, Playdom, SGN, RockYou, Slide and how much venture capital money they have raised so far.