It comes preinstalled in a lot of phones and cant be uninstalled and in some phones they dont preinstall the main fb app instead they’d have something like ‘facebook manager’ or ‘something service’ which only shows up in app list when you select the “show system apps” option.
Thats what internet.org was about, fb partnered with major telecoms in many countries with no net neutrality laws to let people use fb for free and a few other websites but except for wikipedia most were pretty much useless.