Monthly, active according to this source:
https://www.theverge.com/2017/6/27/15880494/facebook-2-billi...
And if you look at these stats from March 2017:
https://newsroom.fb.com/company-info/
It could be either one. Daily active were 1.28 B and monthly active were 1.9 B so either way it is likely they have both crossed, or are about to cross the 2B mark.