Part of why Facebook paid such eye-popping sums for Instagram and Whatsapp is any network that gains critical mass poses a real threat, that monopoly is not as ironclad as it seems.
But not everybody is a sellout and when snapchat refused to be bought, facebook tried to pull a google and release a similar service/software to compete and starve the insolent, but they failed hard and nobody knows what facebook poke is. Though facebook still has its monopoly and snapchat is not really threatening that.
But table turns, and they turn fast. For facebook who has no inherent value other that a lot of registered useds locked in, if those people started migrating to the new thing in town, facebook could be the new myspace in no time. This could not happen to apple or amazon who have inherent value, to google who sells ads outside its own websites, and microsoft well I don't understand how they made it so far, probably because they could bleed money by millions for years without even noticing it.