The people running Facebook don’t live in Spain, that’s 100% why.
I just looked it up and the rankings are a little less black and white than I expected too.
A lot of the numbers seems to be from older rankings and the explosive growth of some languages in recent years makes them vary wildly.
All in all, it seems Chinese is first, with Spanish coming in just after Hindi+urdu, English, and Arabic.
Interesting stuff.
But either way you count Spanish is not #1, but one of the top few.