This is a great article in The Conversation.
“Digital technologies, such as social networks, online shopping, and games, use a set of persuasive and motivational techniques to keep users returning. These include “scarcity” (a snap or status is only temporarily available, encouraging you to get online quickly); “social proof” (20,000 users retweeted an article so you should go online and read it); “personalisation” (your news feed is designed to filter and display news based on your interest); and “reciprocity” (invite more friends to get extra points, and once your friends are part of the network it becomes much more difficult for you or them to leave).”