Playing devil's advocate, for starters, you can consider the effects of having 175 billion dollars (FB's market cap) vanish from the economy, and the effects that would have on confidence in the market.
Not that I disagree with the overall premise that FB doesn't really add anything of much value (or at least anything that couldn't be quickly and easily replaced).
I'll take the "almost none" position, comparable to the season finale of "Friends."