Extrapolated data shows that iPhone and Android have 86 million and 76 million active Facebook app users, respectively. It's also worth noting that Facebook's feature phone app, Facebook for Every Phone app, which has been around for a few months and relaunched just a few weeks ago, already has 7.5 million active users.
Facebook features a separate page for every app authorized to connect to its system, which in turn lists the active users. As a result, you can see how many people are using each Facebook app on each mobile platform, according to Benedict Evans.
Evans compared these numbers to Facebook's latest figures for its total user base (750 million users as of July 2011) and the number of people using Facebook from mobile devices (250 million users as of March 2011). He took into account the many third-party apps, including on the iPad, which does not yet have an official Facebook app, and on Android, where OEMs make their own (and thus there could be overlap with people using both the OEM app and the official app).
Evans found that all of the apps combined add up to 222 million users. That's 88.8 percent of Facebook's total mobile users, though the real percentage is likely smaller as the 250 million number has of course grown since March. The remaining users are people exclusively using the mobile web for Facebook (4 percent in the mobile web only category above). The real mobile web number is likely bigger because of overlap. Either way, Facebook has around half as many mobile users using third-party apps compared to its own official app.