Walking away from AT&T was hard because of the expense of building a phone transmission network, and (I suppose) because of some patents they held.
Walking away from Twitter is much easier, because multiple global instant or near-instant messaging systems exist, including completely open and user-controlled systems, and even building a new such system is reasonably easy for a group of competent people.
That's looking at it from a purely technical perspective. A 20% better Twitter or Facebook (from a technical perspective) is certainly comparatively easy to do, but would still have an approximate value of nil. The value of Twitter or Facebook is not technical (though running services at this scale is technically very challenging): it comes from the people using it. Building another Facebook, even with marginal improvements, is never going to be enough to make enough people switch.
So is the case for AT&T: even if you could build a network competing with the AT&T one it would still have been a largely useless endeavor, unless you could also force AT&T to be interoperable with your network.
Walking away from Twitter is much easier, because multiple global instant or near-instant messaging systems exist, including completely open and user-controlled systems, and even building a new such system is reasonably easy for a group of competent people.