I've actually never heard of MS Spaces before! I guess I don't hang out with that many teenage bloggers. Although I do know the creator of Tabulas, which is an extremely popular platform for Asian teen bloggers.
I think Six Apart shot themselves in the foot with their pricing changes last summer. Not just the fact that they started to cost more (for some people) but the way they rolled it out alienated tons of users.
I had already been looking at WordPress at that point, so the new pricing structure (which would have cost us on OrangePolitics because of course we have more than 3 authors) was all I needed to make me leave MovableTyle.
I can imagine a worse fate for MT than a Yahoo buyout. They've done good work, I'd hate to see them go away.