Facebook doesn’t cause cancer. Those who make such claims while clinging to the professional title of “scientist” are hacks.
There are plenty of bad things you can say about Facebook, this is just not one of them.
If you require more information of the subject Dr. Ben Goldacre has done a great job of laying out proper arguments regarding Facebook and cancer.
I’m not going to write more because I honestly can’t believe that it became necessary for anyone to write a rebuttal to such a ridiculous claim in the first place: http://www.badscience.net/2009/02/the-evidence-aric-sigman-ignored/