When someone studies peoples posts on Facebook (Comments, pictures, status updates) to the point to where they know everything about a persons life, without ever talking to that person. A classic Face-creeper is known to know way more about a victim than the victims best friends know about him.
Phil: "Who knows where Kelsey is?"
Sid: "Oh. Kelsey is soo totally bored while grounded for getting a D+ on her math final."
Doug: "What?!... When did you talk to her? I didnt even know you guys talk that much.."
Sid: "Kelsey added me last week on facebook. Did you not see her latest 8 updates?