Can’t find canonical sources right now, but the percentage of users of Facebook on mobile is 90%+. Of that, most of them would be using the official app, where traditional ad blockers wouldn’t work.
But ad blocking is enough of a money problem for Facebook that it keeps attempting to beat them through many tricks (spelling words with invisible characters, putting words split into characters and each one in a <span>, etc., to beat pattern matching).
The rare times when I use Facebook, I always login through a browser with an ad blocker (and tracker blockers) on.