To become obsessed with Youtube. When logging in becomes an hourly habit, and you find yourself sub-for-sub'ing in order to maximize your online popularity. When you actually make another account to upvote all of your video's ratings, or when you constantly check every 2 minutes to see if your comments have been upvoted or downvoted.
Usually tubescessers are former Myspacer's/Facebooker's that are looking for a new outlet to increase their online social status.
Gerald: Hey want to go play catch in the park?
Billy: *sets UToob to homepage* AHA! Now I can increase my productivity on Youtube!
Gerald: Billy, I think you've become tubescessed.