Twitter Groups allows you to tag your followers into different groups. Then you can send a message to those groups without needing to send the message to each person one at a time!
R. Jäschke, L. Marinho, A. Hotho, L. Schmidt-Thieme, and G. Stumme. Knowledge Discovery in Databases: PKDD 2007, 11th European Conference on Principles and Practice of Knowledge Discovery in Databases, volume 4702 of Lecture Notes in Computer Science, page 506-514. Berlin, Heidelberg, Springer, (2007)
P. Chirita, S. Costache, W. Nejdl, and S. Handschuh. Proceedings of the 16th International World Wide Web Conference,
WWW 2007, Banff, Canada, page 845--854. New York, NY, USA, ACM Press, (2007)