According to various scalability articles on FaceBook, they continually rewrite their software. It's probably been rewritten far more than Twitter has, you just never hear about it because TechCrunch doesn't have a source inside.

In my experience, that's really the only way to do software. It's completely unrealistic to think you'll get it right the first time, and yet completely destructive to go and start from scratch. So you pick your worst problem, rewrite it, then hook it back into the working system without touching the rest. Then pick your next-worst problem, rewrite it, etc.