LinkedIn is a business-oriented social networking site founded in December 2002 and launched in May 2003 mainly used for professional networking. The purpose of the site is to allow registered users to maintain a list of contact details of people they know and trust in business. The people in the list are called Connections. Users can invite anyone (whether a site user or not) to become a connection. (from Wikipedia)
Facebook is a social networking website that was launched on February 4, 2004. The free-access website allows users to join one or more networks, such as a school, place of employment, or geographic region to easily connect and interact with other people. Users can post messages for their friends to see, and update their personal profile to notify friends about themselves. (from Wikipedia)