There's a funny story about when cofounder Dustin Moskovitz first wanted to help on the site. Choice of language was obviously not such a big deal to them.

From http://www.businessinsider.com/how-moskovitz-became-facebook...

>Dustin was like, 'Hey, I want to help out. I want to help you do the expansion.'

>Zuckerberg told Moskovitz, "That's pretty cool. But you don't know any PHP."

>"That weekend he went home, bought the book 'PERL for Dummies,' came back, and was like, 'OK, I'm ready to go,'" Zuck said, and told him, "The site's written in PHP, not PERL, dude."