Twitter is an online social networking service that allows users to send and read short 140-character messages called tweets. Registered users can post tweets while unregistered users can only read them. Instagram is a mobile photo and video sharing service that enables users to take pictures and videos and share them on social networks like Facebook and Twitter. Facebook is an online social networking service headquartered in California, launched in 2004 by Mark Zuckerberg, that allows users to post thoughts and pictures for others to see.