This is a great example of how to mine twitter. My company (www.syncapse.com) takes a very similar approach and had similar findings around the 20% reciprocity on Twitter. Twitter as a 'social' network is not that social. It is important to note this when calculating the earned impressions or media value that your tweets can create. A few axioms ring true; 1)birds of a feather flock together, and 2) pyramid models don't exist.