Twitter added support for "@replies" beginning in May 2008,[3] with any combination of @ with a username being turned into a hyperlink to the profile. On March 30, 2009, Twitter updated the feature and renamed it "Mentions" (i.e., to "mention" user "@janedoe") so as to include non-reply posts directed at individual users.[4]
C. Drummond, and R. Holte. Proceedings of the Seventeenth International Conference on Machine Learning, page 239--246. San Francisco, CA, USA, Morgan Kaufmann Publishers Inc., (2000)