I must say that I was a bit suprised by it - apparently it's about the traction not about the biz model, kinda like delicious' theme investment.
Now, Fred is a smart guy and has seen lots of things but delicious was solving a problem. I can't seem to understand what problem "microblogging" is solving as of yet, especially given that there is a fair number of substitute products. The upside, which I bet was one of the triggers of the investment, is that there's a fairly large ecosystem built around twitter, and Ev's also the kind of guy who can gather a significant community around his product. At least he did it with Blogger.
So I guess the next level challenge is growing that ecosystem and make it attractive for a strategic investor. Quite difficult but not impossible. The chemistry seems right.