Altrag already answered this very well, but I just wanted to make one other point: in lots of industries the consumer can already "defect" to a competitor with absolutely no impedance. It doesn't matter if I'm buying a computer from Dell or a box of Cheerios from Safeway: nothing stops me from buying a computer or cereal from a different manufacturer the next time I want one.
However, the "defectability" possible with all those products hasn't caused Dell or General Mills to go bankrupt; quite to the contrary both companies (and many others) have found ways to make the user want to continue purchasing their goods. Similarly here, even if a social network with almost no cost of leaving were to exist, it wouldn't necessarily mean that any company operating it would go out of business. Instead, it would just mean that company has to figure out how to please its customers.