Thanks for weighing in, Elliot. I routinely go back and clear out many (not all) who don't follow back. As I do this, I think they wasted an opportunity for another set of ears, a customer, and/or a potential champion for their blog, product, or cause. They're also lost the possibility of mentions and retweets from an engaged account with a significant number of followers. I think your point also begs the question... how many RSS feeds can one reasonably follow? I know the number has to be much lower than the number of Twitter accounts one can follow.