Facebook is looking for platform developers to test a payments system, an administrator announced on the Facebook Developers forum. Details are scant, but it's more likely than not built on the micropayments software Facebook developed for its virtual gifts. (Which, by the way, is a brilliant Trojan horse strategy: Charge people a token amount for something that costs you nothing, and get their credit-card numbers while you're at it.) The good news for the rest of us is that the new payment system might mean we'll see some Facebook apps that are meant to do something besides show us ads while we goof off.

But don't get too excited. After all, even the Facebook-app advertising economy is scarcely developed. Most of the apps making money are ... advertising other apps. It's as if this were California in 1849 and you had people setting up picks-and-shovels advertising agencies before the picks-and-shovel merchants opened their doors. Online advertising works, obviously, but no one's hit on the right formula for Facebook apps yet.

Turning Facebook into a marketplace is another potential revenue stream for the site. As eBay learned with PayPal, it's not enough just to list goods; you have to provide the means to move cash and close the deal, too. Expect to see a ton of e-commerce apps — and a rash of alarmist articles predicting Amazon and eBay's doom. There's always a market for those.