Networking engines

One of the wonderful services/functionalities that is offered by Facebook, Linkedin or other social networks is the one that informs you with this message: John S has updated his profile. Just this simple message posted in your landing page, or in your inbox, is powerful enough to make you visit John S profile.
This occurs because we are naturally curious about many things and, specially, the ones referred to people we know. So curiosity is a human emotion driving us to explore the unknown but also a powerful engine which can be used to drive our attention to a specific site.