You cannot copyright a Tweet

Contrary to popular belief and Twitter's terms of service, you cannot copyright a Tweet. Under US law, copyright is granted on publication to "original works of authorship" finalized in "fixed forms of expression" but this does not extend to names, titles, or short phrases (PDF).

As messages sent via Twitter cannot be longer than 140 characters, they cannot be copyrighted. However original, witty, or profound they may be, nothing more than good manners protects your original...