Their old 10Q said this:

> An active user is someone who visited Facebook.com and logged-in (or has been logged in) or who has taken an action with a Facebook feature (for example, clicked a ‘like button.)

The current ones say this:

> a monthly 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), in the last 30 days as of the date of measurement.

Clicking a like button is no longer sufficient to count as an active user. I believe there were articles about this when they changed it, but I'm not sure.