When you're scrolling down your news feed to see what's happened since you last checked and hit photos / statuses that you've already seen. This is followed by a sense of remorse and pity for your perceived lack of social life, tinged by anger at your friends for not posting more new stuff to keep you entertained.
Tristan logged on, saw his notifications, and got excited at the thought of all the new things that must have happened since his last logon. He only got 20 seconds of scrolling in before he hit the deja vu photos and logged off in sadness. This is known as Facebook reburn.