For Facebook? Just make a fake account with some made-up name, up your privacy settings so no one can search for you in the first place (especially important when using a fake name, otherwise you'll get 'is that you?! long time no see'-style messages), and in you go.
"One of the bitches actually gave birth while she was attacking, and her puppies joined in on the carnage."
--the awesomeness that is Boatmurdered.