A recent BBC2 programme The Secrets of Silicon Valley concentrated on Fakebook et al, and one expert on manipulation commented:
"we react to news that has impact" - positive or negative. ie Sensationalism spreads.
Nice news makes us feel warm inside, but is not sufficient to make us disseminate it.

Another aspect is something I learned years ago to my cost, in sales blurb. If it is on a printed page we treat it with more credibility than if it is verbal. Farcebook (aka Fakebook) we believe more because it is written and it is our friends writing (who couldn't possibly get it wrong).

Of course impact is highly dependent on our pecadillos. Hence the cat videos, and cranes falling into the harbour. And religions like politics.