At the time your choices for dynamic server web apps were php or perl. The LAMP stack (Linux/Apache/MySQL/PHP or Perl) was very popular back then (early to mid 00s)

Are you new? PHP was the standard for that type of app at the time.

And that was really bad, although Mark Zuckerberg himself can hardly be blamed for that.

Your point is 100% correct, but for the sake of our discourse please strive to be more polite!

I'd prefer you focus your attention elsewhere

There are numerous criticisms you can level at Zuckerberg, but writing the first version of Facebook in PHP is not one of them.