"Daily Active Users (DAUs). We define a daily active user as a registered Facebook user who logged in and visited Facebook through our website or a mobile device, or took an action to share content or activity with his or her Facebook friends or connections via a third-party website or app that is integrated with Facebook, on a given day. We view DAUs, and DAUs as a percentage of MAUs, as measures of user engagement."
Going back to the first SEC Filing from July 2012. There is ONE KEY DIFFERENCE in definition of a DAU.
Jul 2012 DAU: "...or took an action to share content or activity... ...via a third-party website that is integrated with Facebook." Ref: http://investor.fb.com/secfiling.cfm?filingID=1193125-12-325...
Nov 2013 DAU: "...or took action to share content or activity... ...via a third-party website or app that is integrated with Facebook." Ref: http://investor.fb.com/secfiling.cfm?filingID=1326801-13-31&...