Twitter is a social networking and microblogging service. Users use mobile phones or computers to send and to read messages, called "tweets". For years, tweets could be up to 140 characters long, then doubled in November 2017 to 280 characters. They appear on the user's profile page. Tweets can usually be read by anyone, but senders can also keep messages private. Private tweets only go to their list of friends. Users may subscribe to tweets from other people. This is called "following", and subscribers are known as "followers". As of late 2009, users can also follow lists of authors.