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
When an individual suffers repetitive strain injury (RSI) in the first digit of the hand (thumb) from excessive text messaging.
Bill: 'Dude I wasn't ignoring you'
Bob: 'How do you explain that then'
Bill: 'My texter's thumb preventing me from contacting you!'