According to Benedict Evans' blog, and research for an upcoming Enders Analysis report, almost 40% of Facebook is via a mobile app.
This is based on Facebook's own data which says 37.5% of 800 million total monthly active users Facebook disclosed in September access the social network via a mobile app--that equals about 350 million monthly active mobile users. The data also suggests that Windows smartphones continue to be "insignificant" while iOS, Android, and Blackberry phones dominate with Facebook mobile users:
"Windows Phone remains quite insignificant, though that may change next year as Nokia's efforts come fully on stream. Meanwhile around 70% of RIM's 70m active users have installed the Facebook app. That's a high penetration rate (it comes to around 50% for Android and iOS) on what is supposed to be a corporate product, pointing to RIM's strength in messaging, but also to the way that the mix is shifting away from business customers and towards emerging markets and teenaged girls (in the UK at least)."