Figures released Thursday by Internet tracker eMarketer indicated that more than half of US Internet users log in monthly to Facebook while about 20 million US online adults "tweet."

An estimated 57 percent of Internet users, about 132.5 million people, will log in to Facebook monthly this year, according to eMarketer.

By 2013, 62 percent of Web users and close to half the overall US population will be on Facebook, eMarketer predicted.

The percentage of US Internet users firing off at least one message a month at microblogging service Twitter was expected to grow from nine at the end of 2010 to 11 this year.

By the year 2013, nearly 28 million people in the US will be "tweeting," according to eMarketer.

The eMarketer estimates are "a realistic assessment of actual usage," said eMarketer senior analyst Paul Verna.