“But one of the reasons we’re very lucky is our engineering team has selected to use PHP as the primary development language. That allows us to use a fairly generic server type. So we, with a couple of exceptions, have three main server types and run a fairly homogeneous environment, which allows us to then consolidate our buying power.” —Jonathan Heliger, VP Site Operations, Facebook (in interview with Dan Faber)
I think homogenous horizontal scaling (when possible) is a great idea for operations.
Facebook continues to add 250,000 new members a day. The most Tagged ever registered was 600,000 a day. Both use PHP. I can’t speak for Facebook, but Tagged has never needed more than 200 servers on the web tier.