Someone who creates a fake Facebook, My Space or blogging ID with the intent of gaining someone's trust or getting "friend" status so they can create havoc. The sniper will do things like comment negatively on a photo, post a falsehood, or make rude comments with the intent to either embarass or piss off the other party.
I got on this chick's Facebook last night. I commented on her photo that her boyfriend was a JackBag and his hair looked like he was wearing a helmet. She was pissed and kicked me off! I love being an Internet Sniper!