The kind of mobile ad blockers that work on the system level require root permissions afaiaa. Meanwhile, uBlock Origin is available on Firefox for Android, but being a Firefox extension, does not have the possibility of blocking traffic in other apps.

The amount of people who have rooted Android devices are probably far lower than the likely-also-low number of people using in-browser ad blockers on Android. For iOS devices, the number of people with jailbroken devices I would guess to be lower in relative percentage than on Android. I don't know if even just in-browser adblocking is possible on iOS at all without jailbreaking.

Anyway, point is, while ad blocking might be relatively wide-spread on the desktop among all people (why else would media bother to spend valuable developer time implementing ad blocker blockers?), in-app ad blocking probably is much less common and that's what fb benefits from.