A term stemming from two seperate lexical choices one of which is 'Social' to be defined as relating to human society and its members and 'Flat-line' which is a medical term to show the absence of heart contractions or the disappearance of the rhythmic peaks displayed on a heart monitor.
In this sense the conjoined term means the 'dead sector of society' So to speak : Nerds, emo's, paedophiles and so on, people who are marginalised and ostracised from society.
Nerd 1: Dude I have a girlfriend now
Nerd 2: No way
Nerd 1: I'm heading off the Social Flat-line