I think this depends on how much friction side-loading ends up being. If I can just click a link and hit "run" like I can on a computer, yeah it could be an issue.
If it requires diving in to Settings or connecting the phone to a computer -- Facebook would never abandon the App Store, friction to using side-loading would be too high.
If it requires diving in to Settings or connecting the phone to a computer -- Facebook would never abandon the App Store, friction to using side-loading would be too high.