I just saw an article on cnn.com this morning. http://www.cnn.com/2008/HEALTH/12/05/happiness.social.network/index.html
It states that in a University of California - San Diego study, that happiness spreads more than unhappiness in a social network.
I agree with this, to a point.
I find unhappiness spreads very quickly itself. It's the whole domino effect. Your kids are mad, they take it out on you, you get annoyed and take it out on the clerk at the grocery store, she gets mad and starts taking it out on the customers who come after you, and so on and so on.
With that however, you need a personal connection to make it spread.
With happiness.... you don't really need as much of one. How many times has someone's bad mood online affected me? Not very often, I just mute them. But how many times have I seen someone online clearly enjoying themselves more than they should, and it wound up making me a goofball?