The odd thing about the example of Messenger is that as two separate apps they seem highly coupled. AFAIK you need to sign in with a valid Facebook account to use Messenger, so you're already very likely to have the normal app at that point as I can't imagine anyone who would trust FB for messaging but not everything else. On the flip side, barring a few possible holdouts of people worried about app permissions, I don't know anyone who actively have FB accounts but don't use their messaging service.