Michael Abrash on Valve:

Once Doom had been released, any of thousands of programmers and artists could create something similar (and many did), but none of those had anywhere near the same impact. Similarly, if you’re a programmer, you’re probably perfectly capable of writing Facebook or the Google search engine or Twitter or a browser, and you certainly could churn out Tetris or Angry Birds or Words with Friends or Farmville or any of hundreds of enormously successful programs. There’s little value in doing so, though, and that’s the point – in the Internet age, software has close to zero cost of replication and massive network effects, so there’s a positive feedback spiral that means that the first mover dominates.

Valve: How I Got Here, What It’s Like, and What I’m Doing – blogs.valvesoftware.com