For the first time, comScore is counting PC and mobile users in its rankings of U.S. visitors to Web properties. According to the firm, about 51% of Google's (GOOG) U.S. visitors access it via mobile devices, and about 12% are mobile-only. For Yahoo (YHOO), those numbers are 50% and 13%, and for Facebook (FB), it's 51% and 16%. Microsoft (MSFT) has a lot of catching up to do: its numbers are only 24% and 1%. The mobile shift has been seen as a dual-edged sword for Facebook and Google, though each has made a lot of progress (I, II) in improving monetization.