If you're casting around in the social media sphere for customers, your time may be better spent friending on Facebook than tweeting on Twitter, new research finds. The bottom line is that Facebook is the social media site that has the numbers.

More than half of Internet users in the U.S. were logging on to Facebook at least once a month last year, according to new estimates from eMarketer, a digital marketing media research firm. This year, eMarketer forecasts that 132.5 million U.S. Web users will use the site, which has cemented its position as the world’s top social networking site.

By 2013, 62 percent of Web users and almost half (47.6 percent) of the overall U.S. population will be on Facebook.

By comparison, the researchers said, fewer than one in 10 online adults in the U.S. uses the Twitter microblogging service. By the end of last year, 16.4 million U.S. adults —  9 percent of the adult Internet population — used Twitter.

This year, eMarketer estimates, Twitter will reach 11 percent of Internet users and 16.5 percent of U.S. adult social network users. It's projected that by 2013, nearly 28 million Americans will be tweeting.

Facebook’s dramatic population penetration means that its days of double-digit growth are over; single-digit growth is forecast for 2011. Twitter will post higher growth rates, but still will only reach a minority of the population.

“Our figures confirm that well over half of U.S. Internet users are on Facebook, and the social network’s reach and influence will only increase in the next three years,” said Paul Verna, an eMarketer senior analyst.

The firm’s estimates of Twitter usage were lower than previous forecast in April 2010.

“Since then, Twitter has continued to gain traction, but at more moderate levels than we had expected,” Verna said. “Our updated figures put Twitter usage in a clearer perspective than published data showing hundreds of millions of Twitter accounts, or site traffic stats that include visitors who browse public tweets on Twitter.com but don’t actually use the microblogging service.”

Reach BusinessNewsDaily senior writer Ned Smith at nsmith@techmedianetwork.com. Follow him on Twitter @nedbsmith.