Facebook is filled with smart people. They understand the trends better than we do. As you'll note, 4 of their top 5 Risk Factors relate to the web-to-mobile shift and the problems inherent with that.
From the filing:
#2 - "We generate a substantial majority of our revenue from advertising. The loss of advertisers, or reduction in spending by advertisers with Facebook, could seriously harm our business;"
#3 - "Growth in use of Facebook through our mobile products, where we do not currently display ads, as a substitute for use on personal computers may negatively affect our revenue and financial results;"
#4 - "Facebook user growth and engagement on mobile devices depend upon effective operation with mobile operating systems, networks, and standards that we do not control;"
#5 - "We may not be successful in our efforts to grow and further monetize the Facebook Platform;"