Johan Bollen and colleagues at Indiana University have found that happier people tend to exchange tweets with other happy people, and unhappy people tend to tweet with other unhappy people.
They were just analyzing the content of the tweets, without any background data on the people sending them.
It does make sense to me. I don't tweet much, but I do blog and use Facebook. I know I am much more likely to respond to happy, or at least not unhappy, messages. I have also seen exchanges among blog commenters in which unhappy people respond to one another in long chains. These are aptly called "troll wars." I stay out of them.