I suppose what I meant is that, unlike Apple, there's no cost for a user to start using a competitor. But I suppose switch isn't even really the right word here

I suppose what I meant is that, unlike Apple, there's no cost for a user to start using a competitor. But I suppose switch isn't even really the right word here; there's no reason they even have to stop using Twitter. Basically, if something else did gain momentum, it could happen pretty fast. As opposed to an iPhone competitor, which Apple can see coming and make strategic moves to plug leaks.