The days of double digit growth are over for social networks with some age groups reaching the saturation point reports eMarketer.

Social networking now reaches most internet users in the US with nearly 150 million users, using social networks via any device at least monthly this year, bringing the reach of such sites to 63.7% of the online population. It is expected to reach 164.2 million users or 67% of internet users by 2013.

“With fewer new users signing up, social network users will be more sophisticated and discerning about the people and brands they want to engage with,” said Debra Aho Williamson, eMarketer.

“In 2011, social networks will need to cement their relationships with their users, particularly people ages 35 and older, in order to keep them engaged,” said Williamson. “Marketers and media companies can contribute to this effort by creating compelling user experiences that make people want to stay connected to social networks so they can gain access to experiences, deals or content they may not be able to find anywhere else.”

The biggest users of social networks fall into the 18-to-24 age group, where 90% will use social networks this year. Followed by the 12-to-34 age group with 80% and 45-to-64 age group with 50% usage this year.