Exactly! Facebook continues to develop in PHP because throwing out your existing system and replacing it with new code base (and a new language!) is almost always fraught with risks.

That spreadsheet is not "science" even though it has numbers in it. I can put higher numbers for Python and arrive at a different answer.

I completely agree with this comment: http://news.ycombinator.com/item?id=3366883 elsewhere in the thread.