In hindsight, i think a big part is that myspace's reach extended mostly to early adopter internet users. Early adopters jump on ships early, but also jump off easily too. Once everyone's parents and grandparents jumped on to facebook, that's a much better moat.
Yes, I'm curious as to why DOJ announced Google and FTC announced Facebook actions right before the same group of people who nurtured their rise to monopoly status retake power.
FB buying up the competition probably helped too.