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Facebook is a social networking service and website launched in February 2004, owned and operated by Facebook, Inc.
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LinkedIn is a social networking website for people in professional occupations. Founded in December 2002 and launched in May 2003, it is mainly used for professional networking.
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Twitter is an online social networking service and microblogging service that enables its users to send and read text-based posts of up to 140 characters, known as "tweets". It was created in March 2006 by Jack Dorsey and launched that July.