SuperData, this total marks an 11% increase over digital sales in 2012. SuperData noted that Christmas and New Year's occurring in the middle of the week may have helped spiked end-of-year numbers; December 2013 saw a 36% increase in digital sales compared to December 2012.
Mobile titles claimed the largest percentage of the digital pie, representing $3.6 billion in sales. Free-to-play titles were up 45% year-on-year to $2.9 billion. Perhaps most interesting for MMO players is the fact that F2P MMO offerings from games like World of Warcraft and Star Wars: The Old Republic landed in the top ten, though the top three free-to-play earners in 2013 were CrossFire, League of Legends, and Dungeon Fighter Online.
Data used by the firm is gathered by collecting digital transaction data directly from developers and publishers.
[Thanks to Hagu for the tip!]