Mark Zuckerberg probably choose to build Facebook using PHP because it was, and still is, one of the easiest programming languages to learn and to get something working quickly. Other possible reasons may be that PHP was, and still is, very common in terms of its availability in hosting packages.
Also, there was a lot of documentation available and not a lot of strict standards that needed to be adhered to. Who knows for sure? Maybe someone should ask him. Is it possible he was taking a PHP programming class at Harvard and needed to use PHP in a project?