"Facebook occupies an interesting space in mobile. We’re not an operating system, but we’re not just an app either. Facebook accounts for 23 percent of the time people spend on smartphones. The next-biggest ones are Instagram and Google Maps, which are each at 3 percent. For the past 18 months, we spent our efforts building good versions of Facebook’s mobile apps. But the design was still very close to what we have on the desktop. We knew that we could do better."
Update - Flurry says it's 18% for connected devices in the US (first chart)
Update - I've tracked it down, and Mark Zuckerberg wasn't 100% accurate. It's from comScore data for the US from December 2012, and it says that Facebook accounts for 23% of the time smartphone users spent with apps. With apps. An important distinction!