I don’t have a Twitter account but if I want to have virtual reunion with my friends and have a constant Internet connection with them, I am tempted to try it.
" Twitter is essentially a net-based chatroom filled with your friends. The basic idea is you log in to the service, add friends, and start sending them simple little messages. These messages are limited to 140 characters or less, so you have to make do with a sentence or two. You can elect to send and receive messages any number of ways–through Twitter's website, through your mobile phone (via SMS), through your IM client, through RSS, or through a number of third-party tools that have emerged to help people access and interact with the Twittersphere", explains Fred Stutzman in his The 12-Minute Definitive Guide to Twitter.
In short Twitter builds a web community around you and your friends by asking you to answer the question " What are you doing?" 24/7 with online messages. There is also a public timeline that displays everything everyone of your buddies is up to. Some of the message could be kept private.
And if you wonder what good is Twitter to libraries read David Lee King's "Twtter Explained for Librarians, or 10 ways to use Twitter".