Yesterday comScore reported that Facebook had the number one US mobile app of 2012. It beat out perennial leader Google Maps for the top spot. As an aside, Google Maps now has more usage in mobile than on the PC according to comScore.
There's nothing necessarily surprising or remarkable in Facebook's rise to the top position. It has long been one of the most popular apps and the app with the greatest monthly engagement (time spent).
What may be most significant here is the way in which Facebook's victory over Google operates as a kind of "metaphor" for the differences between PC and mobile usage. Search is not the center of the mobile experience as it is on the PC -- although local search, represented by Google Maps, is a critical and hugely popular function. Of course Facebook Nearby is a local search tool and has significant potential if Facebook invests further and continues to develop it.
Notwithstanding Facebook's win, Google still dominates the top 10 in comScore's chart above.
As widely discussed Facebook's challenge is to fully "monetize" all this mobile traffic/engagement without negatively impacting or "corrupting" the user experience. Search will help the company do that as would a mobile ad network for third party publisher sites that used Facebook user data. (There are some privacy issues and potential challenges there and it's on hold.)
Facebook has "display" ad units for mobile that are helping to quickly ramp its mobile revenues. The company allows mobile-only targeting as well as combined or cross-platform targeting for PC and mobile, thereby simplifying the mobile ad buy for marketers.