Time bombing is when you intentionally comment on an old photo on someone's Facebook profile as if it's current. The older, the better. This forces Facebook to make it appear in the current
news feed and typically prompts awkward comments from
other friends who think they are commenting on new pictures.
When someone is time bombing you, you'll get notifications from friends with comments like:
"When did you color your hair green!"
"
I'm glad to see you're back with
Nancy. That chick you were dating last week was really nasty"
"Wow - are you pregnant AGAIN!"
"OMG - did you lose like 100
pounds? You look great!"