Twitter was always meant for mobile, so the vast majority of Tweets originate from mobile devices. Meanwhile, the number of Tweets from desktop computers is consistently falling [source: Madrigal].
From the beginning, Twitter's founders designed the service to work with the Short Message Service (SMS) protocol. SMS allows you to send and receive text messages from a cell phone to other phones and services like Web sites, voice-mail systems and e-mail servers. When you send a text message from your phone to Twitter, the message transmits to a mobile switching center (MSC), which sends the signal to a signal transfer point (STP). From there, the message goes to a short message service center (SMSC), which then sends the text to Twitter. Twitter sends the message back out to the people in your network using the same process in reverse.
The SMS protocol has several restrictions, which are the source of Twitter's limitations. An SMS message has an upper limit of 160 characters and can't include anything other than text. While there are other protocols that can send more information than SMS, they aren't as widely supported by cell phone service providers. By limiting messages to the SMS format, Twitter is able to reach a larger customer base.
Twitter will also send messages over SMS to cell phones even if you use a desktop or Web-based application to post your Tweet. When you post your message, you tell Twitter to send the message out to all the appropriate outlets through the syndication format. Twitter sends the Tweet out to the cell phones of anyone in your network who has added a cell phone number to his or her Twitter account. For other users, the message may only appear on a Web page or in a computer desktop application.
Twitter members in the United States can interact with the service through their cell phones by sending text messages to 40404. In Canada, the code is 21212, and in the United Kingdom, you'll give your fingers a workout with the code +44 7624 801423. Through text messages, you can subscribe to other members' feeds, turn off feeds, add friends to your network or even delete your account.