Twitter has been granted a patent which describes the basic principles of the service itself.
Originally filed in July 2008 and granted to Jack Dorsey and Christopher Isaac Stone, thepatent describes a system for "device-independent point to multipoint communication."
From the patent: "The system is configured to receive a message addressed to one or more destination users (...) The system applies rules to the message based on destination user information to determine the message endpoints, the message endpoints being, for example, Short Message Service (SMS), Instant Messaging (IM), E-mail, web page output, or Application Program Interface (API) function call."
To us, this sounds exactly like Twitter, and could potentially cause a lot of headache to anyone providing similar service. There's good reason to believe that Twitter will only use the patent defensively, though, as Twitter in April 2012 pledged not to pursue offensive litigation over patents without the consent of the employees who created them.
"Like many companies, we apply for patents on a bunch of our inventions. We also think a lot about how those patents may be used in the future, which is why we introduced the Innovator's Patent Agreement to keep control of those patents in the hands of engineers and designers," said Twitter in a statement to the Verge.
Still, we're sure that Facebook, which offers similar functionality (i.e. you can also send a message to multiple recipients who follow you), is at the very least uneasy about this patent.