I'm not sure there are metrics on that, I haven't seen any publication of such a precedent. I also wonder where those people might go, to the next-biggest platform perhaps? Or create a new one? And what about people that don't have a 1-to-1 relation to presence and consumption but a 1-to-many (i.e., friends, family). I suppose it's likely that someone might spend less total time on a platform if they have to divide their time between more than one platform.