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_ LinkedIn Corp.

The professional social network was launched by Reid Hoffman, one of Facebook’s early investors, in 2003. While Facebook, which got its start a year later, was meant as a place to connect with friends and family, LinkedIn is strictly business. The company went public in May 2011, the first of the latest crop of Internet companies to do so. As of March 31, LinkedIn had 161 million users. Its profit more than doubled in the first three months of the year, helped by revenue from the fees that it charges companies and recruiters to get broad access to its site.

_ Twitter Inc.

Launched in 2006, Twitter lets its users express themselves in short messages _ tweets _ of 140 characters or less. It’s a cascade of information, and unlike Facebook’s complicated privacy controls, there are just two choices: Public or restricted to pre-approved followers. Celebrities, politicians and companies use Twitter to keep in touch with fans, constituents and consumers. It has more than 100 million users.

_ Google Plus

It wasn’t Google Inc.’s first foray into social networking, but it was the search company’s most ambitious try, designed to go head-to-head with Facebook when it launched in 2011. Although it has some 170 million users, including some Facebook holdouts, Google Plus has, so far, failed to draw a substantial number of people away from Facebook. Google uses the service to plug social elements into search and its other products.

_ Pinterest, Instagram and the future

Facebook’s success has spurred a new class of social networks that are smaller and more specifically focused. Instagram, which Facebook plans to buy for $1 billion this year, exists only on mobile devices and only to share photos., which launched last year, aims to collect people who are physical neighbors through a closed social network. Path, founded by ex-Facebook employee Dave Morin, dubs itself a “personal network,” letting its users share photos, videos and updates with a smaller group of friends than on Facebook. Pinterest, another photo-heavy site, is meant for sharing images. Users often collect images to help inspire ideas for projects, like a garden, or events, such as a wedding.