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Sorry Facebook... PHP is dead. But, that's okay, because I would recommend Ruby anyway... much cooler IMHO. I've coded extensively with both.
Actually, if you really want to know the way to go... It's HTML5 + CSS3 + JavaScript. And then Node.js with Express.js for a very light-weight server that just serves data. And MongoDB and/or Redis for persistent storage. The application & business logic (and in many cases, storage, esp. now with HTML5) should go mostly in client-side JavaScript as much as possible. This will help the app scale by taking load off the server.
So, JavaScript is really the language to learn. I recommend JavaScript the Good Parts for object-oriented JS. Helpful frameworks are jQuery, and I'm also checking out backbone.js. looks cool.
There really aren't many books on Node.js yet, but there's a lot on the web about it.