How much money can Facebook apps actually make?DeveloperAnalytics, a research firm which analyzes Facebook applications, put out an appealing bit of linkbait this morning that purports to show how much money popular applications could earn each month. It calculates the metric based on "hundreds of real CPM, and CPA/Virtual Goods revenue data points collected directly from developers and partners." That's CPM as in "cost per thousand" — the traditional way ads are sold, based on the number of people they reach — and CPA as in "cost per action," which is usually based on linking payment for an ad to its generation of sales, signups, or other results. Virtual goods? Those are the cheesy little icons you can send your friends on Facebook. Yes, some people pay money for them.The list is topped by an widget called Mob Wars, which exhorts users to "Join the Mafia, and start your own mob. Band together with your friends to become the most powerful force in the elite criminal underworld of Facebook." DeveloperAnalytics says Mob Wars' users return to its page 60 times a day. Facebook's most popular application, Slide's FunWall, only shows up fifth on the list, because users load its pages just two or three times a day. Here's what DeveloperAnalytics didn't account for in running the numbers: Slide's opening an office in New York to sell its inventory to major brands, while Mob Wars ads ask if you want to take an IQ challenge.