You are considered an "active user" even if you don't use Facebook but a 3rd party uses your Facebook ID to login or share, via the third party service.
If you use the messenger app at least once in 30 days, you're a Facebook "active user" even if you haven't logged into Facebook for 10 years, but still have a Facebook profile.
I use my Facebook ID to login to Disquss, and I haven't visited Facebook in over a year. Facebook considers me an "active user" of Facebook.
"We define a monthly active user as a registered Facebook user who logged in and visited Facebook through our website or a mobile device, used our Messenger app (and is also a registered Facebook user) or took an action to share content or activity with his or her Facebook friends or connections via a third-party website
or application that is integrated with Facebook, in the last 30 days as of the date of measurement."
Facebook Changes Definition of Monthly Active Users
Guess why Facebook changed its definition of an "active user" to be extremely broad and include people who never use Facebook anymore.
Guess why Facebook got busted hiring an Indian company to click on people's Facebook ads.
Guess why Facebook "miscalculated" , by 80%, the amount of time people spend watching videos.
Facebook Overestimated Key Video Metric for Two Years
Facebook is shady as fuck and constantly misrepresenting their numbers in BIG ways because Facebook is dying.
For $200 bucks you can buy 2,000 active Facebook profiles.