I honestly laugh whenever I hear people still bring up those early social media companies as evidence that social media heavyweights somehow come and go. None of those companies ever came near the global reach of FB. A quick Google search gives a peak user count of MySpace of between 75 and 100 million, for FB it's 2.7 billion.
Those companies basically all existed before the vast majority of people were even on any social network, and importantly none of them had real-name policies initially. And the modern challengers you give all have some specific, much smaller niche, e.g. image galleries for Pinterest. There is simply no other social network that has ever challenged Facebook in the "connect all my friends and let me give and see life's updates" space.