@BenBrocka cstheory was also once young and small, although I do agree with your later comment that cstheory had the benefit of existing user-base. It was not however from SO, it was from
MO. A good thing to understand, would be how MO started and gained such an amazing research scope. They also had an easier starting point that us, in that there was already a culture of blogs and other online media in research math that I don't think is present in cogsci.