The experience of having a conversation with someone about something you already learned from reading about them on the internet.
Like déjà vu, which means 'already seen' in French, déjà lu means 'already read.'
A: I finally talked to that Maura girl I've been stalking on Facebook.
B: Is she single now?
A: Nah. I asked her if she wanted to get a drink some time, but she told me she had a boyfriend named Joe. It was total déjà lu. I made some definite headway when I mentioned Haruki Murakami, though.
A: Party tomorrow at 14 Catherine St!
B: Déjà lu, dude. I already RSVPed on Myspace.