Twitter is a messaging service is a short sentence can be sent, it says that you are doing it, the user can send messages no longer than 140 characters.
Twitter is a short messaging service is a sentence that can be sent to you. What are you doing at the time. Users can send text messages longer than 140 characters.
Twitter is the messaging service is a short sentence can be sent to a said. What are you doing at the time. Users can send messages are no longer than 140 letters.