From the link in the post Facebook has two different metrics: MAU (month average users) and MAP (monthly average people). They don't define these terms but MAU sounds like someone with a Facebook/IG/WhatsApp/etc account who logs in, which that 2.74B figure applies to.
The terms are defined somewhere in their reports (maybe only the annual ones).
My understanding was that you are counted as an MAP if you log into FB on any device, or use fb-login for at least one service in that month (although they may have changed it).
