I believe Python might have worked. It's grown well as a web development language since then by itself. It may have forced several rewrites before WSGI came about. It is a relatively easy language to learn, to hire for, and has few massive warts. (Ruby is roughly in this category too, in my opinion.)

I believe Perl probably would not have worked. It hasn't grown much as a web development language since then (although it has some great web frameworks). The language itself has languished in both development and community growth, and it fairly esoteric and certainly harder to learn than Python and PHP, and would have been harder to hire for. It seems unlikely that pre-2010 Facebook to meaningfully have changed the trajectory of the language (ie, Perl 6 being a success).

I don't think Java, C++ (or C), Cold Fusion, ASP, Haskell, Ocaml, and many other languages would have worked, for a bunch of reasons. Harder to hire for, harder to iterate with, more costly in terms of infrastructure, harder to teach/learn, and so forth.

PHP isn't a totally insignificant part of Facebook being as successful as it has been. A good-enough language choice was necessary, at least, and PHP it seems was a good-enough choice.