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I, of course, found it quite interesting that Facebook is running PHP.
You might have noticed that the user-facing portion of Facebook is written in PHP, and we're fortunate enough to be able to make contributions back to some PHP related projects.
They even allow php developers (mostly college students from what I've seen) create custom applications [developers.facebook.com], something that seems to be very popular among facebook's users.
Overall their contributions to the php community are nice additions.