"So are you on Facebook, sir?"

Student: "So are you on Facebook, sir?"

Mr Zuber: (incredulous look) "Of course I am!" (do you think I'm a total loser?)

Student: (impressed at Mr Zuber's coolness) : "Oh wow! Can I be your friend?"

Mr Zuber: (incredulous look) "Why would you want to be a friends with a teacher?"

Student: "But I want to see who all your friends are!"

Mr Zuber: "But they are all maths teachers!"

Student: "Hmm.. yeah .. maybe not...."

I've found this approach works wonders dissuading students asking the perennial Facebook question and beats having to explain teachers can't and shouldn't do that - which they knew before they asked the question anyway :-)