Bob Kraut is discussing some of the data analysis issues in his study in Usenet newsgroups of what independent variables predict whether a message would get responded to.
They first did some machine learning techniques to identify the signature of messages that have a "self-introduction". Then they used that as a regressor, along with some directly measurable variables like using first-person pronouns.
He and Moira Burke have a paper tomorrow where they did a controlled experiment. They found that the key ingredient is saying that you're part of the community, not that you share the interest/condition around which the group has formed.