(Julie Jacobson/ASSOCIATED PRESS)

In its social media usage survey released Monday, Nielsen said that in May 2011, the number of Internet users 55 and older using social media sites over mobile devices grew by 109 percent from a year earlier.

That age group was particularly active on professional sites such as LinkedIn, where one-third of all mobile app downloads are by users 45 and older, according to Radha Subramanyam, Nielsen’s head of social media and advertising analytics.

Facebook’s largest mobile user group is between ages 25 and 34, she said.

The older age group is active on all social media platforms and are using the sites most active for professional networking, she said.

“Boomers are at the forefront of change because they use social media like everyone else and in the context of the larger economy they are the ones that have kept their jobs and are better off so they can afford smartphones,” Subramanyam said.

According to results from the online survey of 1,865 adults, more Americans are using social media sites and are using them for longer periods of time.

Four in five are visiting social networks and one out of four minutes online is on one of those sites. Americans spend more time on Facebook than any other U.S. Web site, according to Nielsen.

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