I think fickleness is more closely related to switching costs than sunk costs. In other words, I don't pay a dime for Gmail, but it would be quite difficult to move away from. Facebook probably doesn't have quite that much of a lock on your data, but I guess that depends on who you are and how you use it.
Everyone would easily switch to a better mail system that offers some kind of a one-click migration/import from GMail, provided it is really better in some ways. In fact, however harsh it may sound, GMail can be ruined in a matter of months just because the majority of its user base is not loyal. Compare it with Apple in terms of loyalty, if in doubt.
