Usability 
Facebook, the most visited site on the Internet, believes strongly in the mobile Web. Facebook.com uses progresive enhancement to provide an optimal experince on everything from the WAP browsers of basic feature phones to the most advanced HTML5 compliant smartphone browser to the desktop.
Facebook's mobile traffic is growing expontentially. In 2011, 350 million of Facebook's 800 million registered users accessed the site using mobile devices. Saying that Facebook is becoming a mobile company, Erick Tseng, Facebook's director for mobile products projects that within a year or two the majority of Facebook visitors will access the device through mobile devices.