Pastbook(ing) is a forming of Internet Trolling where you go through someones Facebook feed and like and comment on a bunch of old embarrassing stuff. This games the Facebook algorithm, surfacing this content on the newsfeed of the target's friend humiliating them.
New Facebook Comment on a 2 year old post that was optimistic about something that really didn't pan out.
S - "Remember when this was a thing? #PASTBOOKED"
A - "NO NO NO"
F - "LITERALLY... my entire feed is you A."
A - " *throws myself in front of a train*"
F - "Looks like a classic case of pastbooking"