What is Facebook?
Facebook is a social networking service and website launched in February 2004. Users must register before using the site, after which they may create a personal profile, add other users as friends, and exchange messages, including automatic notifications when they update their profile. Additionally, users may join common-interest user groups, organized by workplace, school or college, or other characteristics, and categorize them.
What is Twitter?
Twitter is an online social networking service and microblogging service that enables its users to send and read text-based messages of up to 140 characters, known as "tweets".
What is LinkedIn?
LinkedIn is a social networking website for people in professional occupations.