SAN FRANCISCO -- Social networking is snowballing.

That's the message from the latest Nielsen research -- which shows that Americans devote six hours a month, or almost a quarter of the time they spend on the Internet via their personal computer -- on social networking sites and blogs. That's up from 16 percent a year ago.

Driving this trend is the social networking juggernaut Facebook, which recently celebrated reaching half a billion users worldwide. It claimed an 85 percent share of social networking use; News Corp.'s MySpace came in a distant second with a 5 percent share; and Twitter with just 1 percent.

For the first time, games beat out e-mail as the No. 2 online time-killer, accounting for 10 percent of time spent. Nielsen found that half of all Americans online spend time playing games like online-gaming Zynga's explosively popular FarmVille. It was unclear if time spent playing games like FarmVille on Facebook significantly influenced how much time people spend on Facebook.

It's not that people are not e-mailing or using instant messaging. They are engaged in those activities on services such as Facebook. And they are still heavily involved in those activities while online. E-mail was the third-most popular activity at 8 percent, and instant messaging came in fifth at 4 percent. It remained dominant on mobile devices, up to 42 percent from 37 percent.

"We are not going online and doing anything fundamentally different," said Dave Martin, vice president of primary research at Nielsen. "We are going online to communicate, get information, do research, stay abreast of the news and be entertained. We're doing the same things we have always been doing online. The way we are getting to that content is shifting. It has become word-of-mouth. The power and recommendation platform of social networks like Facebook and Twitter are now driving that personalization of content consumption. Social networking is not just about connecting people. It's a new way to distribute all types of content."

The other activity on the rise is watching videos and movies on YouTube, Hulu and Netflix, which jumped 12 percent to account for nearly 4 percent of all time spent online, or an average of three hours and 15 minutes.