Matthew Ingram nails it:

Setting up a site like Truemors is hardly rocket science. I built one based on the open-source Digg clone Pligg, and it took me about four hours to configure on my server — and it definitely cost me a lot less than $12,107. Did it have any value? No. The only thing that could possibly give Guy’s site some value is if it spontaneously developed a thriving community the way Digg and Slashdot and Metafilter have.

The value isn't in the software, it's in the ecosystem around it. Ideas are two-a-penny and easy to copy, but cloning a community is impossible. Winning over hearts and minds is desperately hard and people get attached to their communities. The more time they spend with them, the harder it becomes to walk away from that emotional investment. The scarcity here is attention: you only get to spend your time once.

This gives Digg, Facebook, MySpace, eBay and the like incredibly defensible positions. It's a good bet that the only way these companies can lose their power is by screwing up. Perhaps they'll forget why their members love them, or they'll put obstacles in their way. Building a clone isn't going to win over the community on features alone.