What are the precise definitions of daily and monthly active users? To be a daily user, do you literally have to use Facebook every day for a month?
To get the ratio of DAU to MAU, you can either take the average DAU for each day of the month, and then compare that to the MAU, or you can take the DAU to MAU ratio for each day and then average that. They'll both come out about the same (doing it the second way smoothes it out just a bit more because of the loss of precision which makes the month to month comparisons or quarter to quarter a little smoother).
Otherwise it be easy to game
My interpretation of that would be that if, prior to that change, you logged to spotify with facebook and didn't uncheck the 'share' button you would have been counted, but after you would not have been.
I do find it rather curious that they simply count any user that logs in and 'visited' facebook. i.e. that no 'actions' need to take place or there isn't any metric for time on site required. Seems rather easy to game that number by just not going after bot accounts.
But at the end of the day there's one number that matters: people click ads and buy products -> companies buy ads and pay fees. And it's been going up steadily for the past decade.
If you strongly believe that you've found an inefficiency in the market please feel free to short FB.