See LinkedIn.

A business-oriented social networking site that enables users to connect with colleagues, look for a job or business relationships and get answers to industry questions. LinkedIn users invite people they know and trust to become "linked in" to them, and the business connections of invited users are in turn linked. However, in order to reach connections down the line, requests for introductions have to be made.

Officially launched on May 5, 2003, the five founders invited 300 contacts to become members and wound up with 4,500 by the end of the month. By year end, more than 80,000 members were linked in, and six years later there were more than 35 million members in 200 countries.

LinkedIn Groups
A major feature of LinkedIn is its groups, enabling anyone to start a group based on an association or industry topic. For example, there are tens of thousands of academic and corporate groups that enable alumni and employees to stay in touch. LinkedIn members request an invitation to the group and can receive postings by other members via e-mail.

Premium Services
Starting in 2005, paid premium services were added for dedicated users such as recruiters and researchers that are based on monthly transactions. For example, the service level determines usage limitations such as the number of results per search, number of daily or weekly alerts and number of requests for introductions.