Although Facebook (FB) CEO Mark Zuckerberg denies his company is building a smartphone, working to optimize the world’s largest social network for mobile is high on his to-do list. Speaking with Matt Lauer from the TODAY Show, Zuckerberg said that more smartphone users are actually accessing Facebook through their mobile browser than through apps specifically built for iOS or Android. “There are more people doing that than the iPhone and all of Android phones combined, right? So it’s actually a pretty diverse ecosystem,” said Zuckerberg. Facebook recently released an overhauled version of its iOS app, dumping HTML5 for native code.