I just read an interesting article from sixrevisions.com. The title of the article is Viral Content: Why We Share Some Things and Not Others. They quote research on this topic and share facts, for example, The New York Times has at least 1 article tweeted every 4 seconds.
The article refers to Maslow’s Hierarchy of Needs and claims that the top 2 tiers of hierarchy explain why people share: “People want the respect of others, and they want to feel a sense of belonging.”
Here is a very interesting study from the New York Times’ Consumer Insight Group if you’d like to read more.