Matt Cutts on the scarcity of open source services:

Open source is really good at creating products. Almost any commercial software package or product like Word, Excel, Windows, or Photoshop has a great open source equivalent. However, open source has been less successful at creating services. Where’s the open source version of Google, or Facebook, or Twitter, or Gmail, or Craigslist?

I (unsurprisingly) think the answer looks a lot like WordPress and Automattic, with a combination of distributed and centralized systems complementing each other.