Mobile-only social network MocoSpace announced today that it generated more than 1.5 billion pageviews in May. That’s a larger number than rival MySpace Mobile’s 1.4 billion pageviews for the month of March.

MocoSpace’s traffic grew 50 percent, compared to just four months ago.

This is in an indicator that mobile-only social networks can not only continue to thrive, but may experience an accelerated growth. After news of the recent growth of MySpace Mobile and Facebook Mobile, there was debate over whether the established online social networks will dominate mobile from now on.

MocoSpace is the largest social network designed specifically for mobile phones in the U.S. It also announced another milestone today as it passed the 3 million mark in registered users this month. The milestone follows a May report by browser company Opera ranking MocoSpace as the third most trafficked mobile site in the U.S. after MySpace and Google.

“What we’re witnessing here is a sea change in social networking behavior. It’s moving from the web to mobile at breakneck speed, simply because people carry their phones 24×7. They don’t want to be sitting at a PC to be able to connect with their friends,” said Justin Siegel, cofounder and CEO of MocoSpace. “They’re coming to MocoSpace because it was built from the ground up for easy mobile use.”

This mobile social networking trend is likely to accelerate as well. Some 800 million people worldwide will use social networks from their mobile phones by 2012, a recent report by eMarketer indicated. That will be nearly 20 percent of all mobile phone users

The relatively high number of generated page views compared to the user numbers may be an indicator that MocoSpace users are particularly active on the network. The average member comes to the site more than once a day.

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[MG Siegler contributed to this story.]