LinkedIn is a business-oriented social networking site. Founded in December 2002 and launched in May 2003, it is mainly used for professional networking. As of 1 January 2011 (2011 -01-01), LinkedIn reports more than 90 million registered users, spanning more than 200 countries and territories worldwide. The site is available in English, French, German, Italian, Portuguese and Spanish. Quantcast reports Linkedin has 21.4 million monthly unique U.S. visitors and 47.6 million globally.

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.

This list of connections can then be used in a number of ways:

• A contact network is built up consisting of their direct connections, the connections of each of their connections (termed second-degree connections) and also the connections of second-degree connections (termed third-degree connections). This can be used to gain an introduction to someone a person wishes to know through a mutual, trusted contact.
• It can then be used to find jobs, people and business opportunities recommended by someone in one's contact network.
• Employers can list jobs and search for potential candidates.
• Job seekers can review the profile of hiring managers and discover which of their existing contacts can introduce them.
• Users can post their own photos and view photos of others to aid in identification.
• Users can now follow different companies and can get notification about the new joining and offers available.
• Users can save (i.e. bookmark) jobs which they would like to apply for.

The "gated-access approach" (where contact with any professional requires either a preexisting relationship, or the intervention of a contact of theirs) is intended to build trust among the service's users. LinkedIn participates in EU's International Safe Harbor Privacy Principles.

LinkedIn also allows users to research companies with which they may be interested in working. When typing the name of a given company in the search box, statistics about the company are provided. These may include the ratio of female to male employees, the percentage of the most common titles/positions held within the company, the location of the company's headquarters and offices, or a list of present and former employees.

The feature LinkedIn Answers,  similar to Yahoo! Answers, allows users to ask questions for the community to answer. This feature is free and the main difference from the latter is that questions are potentially more business-oriented, and the identity of the people asking and answering questions is known.

The searchable LinkedIn Groups feature allows users to establish new business relationships by joining alumni, industry, or professional and other relevant groups. LinkedIn groups can be created in any subjects and by any member of LinkedIn. Some groups are specialised groups dealing with a narrow domain or industry whereas others are very broad and generic in nature.

Another LinkedIn feature is LinkedIn Polls.

A mobile version of the site was launched in February 2008 which gives access to a reduced feature set over a mobile phone. The mobile service is available in six languages: Chinese, English, French, German, Japanese and Spanish.

In mid-2008, LinkedIn launched LinkedIn DirectAds as a form of sponsored advertising.

In October, 2008, LinkedIn revealed plans to opening its social network of 30 million professionals globally as a potential sample for business-to-business research. And, in doing so it's testing a potential social-network revenue model-research that to some appears more promising than advertising.

In October, 2008, LinkedIn enabled an "applications platform" that allows other online services to be embedded within a member's profile page. For example, among the initial applications were an Amazon Reading List that allows LinkedIn members to display books they are reading, a connection to Tripit, and a Six Apart, WordPress and TypePad application that allows members to display their latest blog postings within their LinkedIn profile.

In November, 2010, LinkedIn allowed businesses to list products and services on company profile pages; it also permitted LinkedIn members to "recommend" products and services and write reviews.

In January 2011, LinkedIn acquired CardMunch, a mobile app maker that scans business cards and converts into contacts. LinkedIn plans to integrate this functionality into their services in the near future.

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