I feel like MySpace never quite penetrated the mainstream the way twitter has. It was always something people were largely aware of, but outside of the segments it did really well in, people rarely engaged with it.
Twitter seems to be a bit different in that regard.
IMO user engagement is one of twitter's biggest issues.
As it stands right now, twitter is a great tool for journalist research and other niche research. That's been true for a few years now. The problem is twitter has done little to break beyond it OR to completely own that space. It's similar to if facebook never really broke out of its college roots or had high account abandonment rates for non-college crowd. Both of those were proven wrong for fb; fb long ago made critical product decisions to break beyond college(such as complete removing course listings) and working country by country to ensure penetration and engagement.
Back then the internet hadn't penetrated the mainstream like it has now where we're always connected. Relative to that I think they're not incomparable.
Twitter seems to be a bit different in that regard.