Anyone who suggests that social networks are dominated by young people will slowly, but surely, discover this is no longer the case.

According to a recent Pew Internet study, the number of online users over 50-years-old using social neworks nearly doubled to 42% in May 2010 from 22% in April 2009, while social networking usage in the 50-to-64-year-old demographic soared 88% to 47% from 25%. Meanwhile, social networking use among users ages 18-29 grew by 13%—from 76% to 86%.

“Young adults continue to be the heaviest users of social media, but their growth pales in comparison with recent gains made by older users,” said Mary Madden, a senior research specialist and author of the report.

“Email is still the primary way that older users maintain contact with friends, families and colleagues, but many older users now rely on social network platforms to help manage their daily communications.”

While the numbers suggest the demographics of social networks are starting to even out, a far more interesting angle is how the growing number of “seasoned” social network users will change how social networks operate and, as important, how advertisers and marketers approach social networks.

One of the changes that might happen is how social networks attract and retain older users. Rather than touting new features, some social networks may target older demographics by putting the spotlight on straightforward benefits.

There may also be an opportunity for third-party service providers to offer services that are strong on usability and user-friendliness as opposed to having lots of frills. For example, a new wave of services could tap Twitter’s API to create Twitter Lite.

For marketers, the increased presence of 50+ social networking users means more people with disposable income. This could boost the attractiveness of social networks for advertisers looking to reach consumers in different ways.

Another offshoot could see more companies climb on the social media bandwagon because senior executives will no longer be able to claim that social networks are only for young people.

The “greying” of social networks is a natural part of how the market is evolving. There is little doubt it will have a major impact on how companies and advertisers operate and make money.