"The numbers for our key metrics, our DAUs, MAUs, and average revenue per user (ARPU), do not include Instagram, WhatsApp, or Oculus users unless they would otherwise qualify as such users, respectively, based on their other activities on Facebook." -> This clarifies that they are measuring users without considering any Instagram, WhatsApp or Oculus uplift.
"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 used our Messenger application (and is also a registered Facebook user), on a given day." -> This clarifies that they only count if the login happened that day or they did a visit during that day. Not passive open tabs.
It does also have this note "For example, while user-provided data indicates a decline in usage among younger users, this age data is unreliable because a disproportionate number of our younger users register with an inaccurate age." -> This might be some of what people are experiencing. A shift in demographics to older people.
[1] https://www.sec.gov/Archives/edgar/data/1326801/000132680117...