Twitter is a micro-blogging tool that allows short updates (or tweets) to be constructed of 140 characters or less. An update will include the name of the account holder, prefixed with the @ symbol and may also include a hyperlink if entered by the user. A tweet or update can be viewed by anyone unless the account is restricted to those who request access to view the account. No registration or login is required to view tweets, but in order to reply to them or to receive regular tweets as they are published then a free account is required. Launched in 2006, Twitter has become a popular tool for providing short pieces of information or opinion across a variety of sectors.
The number of twitter users is estimated at 12 million users in the United States alone although there is no data to suggest how many accounts are in active use, the frequency or regularity of account usage, or indeed the content of the short messages that are created. Nielson Online reported in March 2008 a total of 14 million US visitors to twitter, Emarkerter.com calculated 6 million US users in 2008, rising to 12 million in 2009 and 18 million in 2010 up to 288 million monthly active users in 2015.