Facebook didn't choose PHP. They continued to use it, as there was a lot of legacy to port if they wanted to.
These people actively measured (using a spreadsheet at that) the various parameters of a programming language and settled the one that fits their requirements.
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.
These people actively measured (using a spreadsheet at that) the various parameters of a programming language and settled the one that fits their requirements.