The growing popularity of microblogging backed by services like Twitter, Facebook, Google+ and LinkedIn, raises the challenge of clustering short and extremely sparse documents. In this work we propose SMSC - a scalable, accurate and efficient multi stage clustering algorithm. Our algorithm leverages users practice of adding tags to some messages by bootstrapping over virtual non sparse documents. We experiment on a large corpus of tweets from Twitter, and evaluate results against a gold-standard classification validated by seven clustering evaluation measures (information theoretic, paired and greedy). Results show that the algorithm presented is both accurate and efficient, significantly outperforming other algorithms. Under reasonable practical assumptions, our algorithm scales up sublinearly in time. Copyright is held by the author/owner(s).