Short for "Social Networking Artificial Face eXchange." This is when someone fakes being someone else on a social networking site. Typically done to get the someone else in trouble, start a fight or just trash the someone else's reputation.
Dude: "Hey, why did you say you were showing up last night on Facebook and didn't show?"
Chic: "I didn't say that on Facebook. I wasn't even on Facebook last night."
Dude: "I bet your roommate that you got in a fight with last week snafx'ed your Facebook."
Chic: "That bitch!"