According to a recent report by Pando Daily, Facebook logs more active users on its mobile web app monthly than it does on its native Android and iOS apps combined. The featured graph, shown during a press meeting at the Facebook headquarters, lacks specific numbers, but the company pointed out that the bars are to scale.
iOS slightly beats out Android in active monthly users, and mobile web uses obviously take the cake - it should be noted that the "Web" field does include mobile web browsers accessed via Android/iOS devices. Basically, even on platforms where Facebook offers native applications, users still like to access the site plainly through a browser. Essentially, those developing a web service need to incorporate a proper mobile web app.
A recent study has shown that mobile internet usage has almost doubled in the last year, from 4.3% to 8.5%. While iOS beats out Android regarding Facebook access, more Android users got online in general in 2011. Web access via a mobile device is expected to exceed 10% in 2012.
The above chart shows the generalized trend regarding mobile OS market shares, ending last year. With RIM on the out, it can be assumed to that Android and iOS will take up the largest part of the market, as mobile web access gains on desktops.