Lostbooking is when someone is reliving Lost (or watching it for the first time) and constantly pausing it to update facebook statuses regarding the emotional rollercoaster that they are on.
This is often followed my Lost veterans or other fanatics commenting and starting a whole thread that ends up going off topic
Person A: (on facebook) Lost almost made me cry and it's not even the end of season 1. But it's so good so I kept watching.
Person B: Who died?
Person A: You already spoiled it for me enough if i say it i'll spoil it for other people
Person B: Hahaha that sea monster thing wasn't even real
Person A: Shut up.
5 mins later...
Person A: (facebook status) UGH I cannot believe I just went from sad to angry in 5 minutes from watching Lost.
Person C: Are you watching it right now?
Person A: yuup
Person C: You're definitely lostbooking