Twitter's rapid growth has occurred largely due to celebrities. But celebrities don't tweet for free. Increasingly they are seeking to "monetize" their tweet streams by including implicit advertisement in their twitter messages. They are also, as Charlie Sheen appears to be doing, hired firms to help manage their social media presence, including interns who often do the writing. In Charlie's case, his ad for an intern specified that the intern must have "tiger blood." Click on this link to read a Washington Post story about the increasing commercialization of celebrity tweets.