Open Source Drives Web 2.0
I'm totally not surprised that, according to this article, Web 2.0 is built on Open Source, the biggest/most visible Web 2.0 companies are built and hosted on open source software platforms.
Think about it: if you've got some crazy idea for a new web-based business, you have the choice of going out and spending kilobucks (to start) for commercial proprietary software--or doing your preliminary development/prototyping on no-cost open source software. If you choose the open source option, you're out nothing but time if your idea flops, so it's pretty easy to make that decision.
Let's say your idea starts to take off, though: with proprietary software, you've got to keep going back to the vendor to buy more and more licenses as you grow. If you'd started out with open source, though, all you need to scale is more hardware (which you need with the proprietary solutions as well). Again, it's pretty obvious that you do better with open source software, especially if you can't drive a revenue stream through your website until you've scaled up.