1) What Accel did by investing in Facebook was the equivalent of spending $100 on a old painting at a yardsale that later appraised on Antique Roadshow for $5000. That was BEFORE the creation of this developer network. The way they've built this thing is like coming home and discovering that, hidden in the frame of your old painting is one of ten known original copies of the Declaration of Independence.
What I'm saying is that, a few years ago, Accel invested a lot of money in a very hot social networking property. However, there's no way in hell, and no way that anyone could really convince me, that they foresaw Facebook's eventual prominence in the social application ecosphere as the OS. I'm sure they just saw pageviews and signups and all sorts of nice charts with hockey sticks on them and wanted to be a part of that. Now, maybe this is what Mark Zuckerberg had in his head... that's a possibility... I don't know the guy, but I seriously doubt he pitched the social OS concept to them before anyone really even gave any thought to what that meant.
2) If Microsoft wanted to spent that $6 billion of agency M&A money more wisely, they would have bought their way into the web OS by buying Facebook for half that. Google, Adobe... someone who wants to be in that layer should seriously consider throwing $3-5 billion their way.
Actually, to be honest, I think the acquirer that makes the most sense is Amazon. Amazon store+S3+Facebook would mean that you have a place to host your app, spread your app, ways to sell stuff in a whitelabeled way through your app, and a social network to integrate into a "Your friends bought this" concept. Imagine how powerful it would be if when I buy stuff, all my friends get notified.
Jeff Bezos... are you listening? Buy Facebook!