When someone on Facebook comments on a picture or status update well after the thread has long been considered dead, buried, and put to sleep. Facelaggingers are easily spotted by total disregard of conventional time or space constraints. Birthdays from 5 years ago, "Happy bday!" Girlfriend you already broke up with, "cute picture!" Trip you went on in college, with you now married with kids, "OMG you were in Italy!? call me."
Facebaiters often are facelaggers, but they are two very distinct disorders of the social mind.
Amy just pulled some serious facelagging on my high school graduation picture, did she just get back from the future with Marty Mcfly?