The majority of mobile phone users are inseparable from their devices, taking their smartphones and traditional phones with them all the time.

This year, 91.4 percent of the U.S. population is a mobile phone subscriber. That number is expected to increase to 95.3 percent by 2011 and again to 96.7 percent by 2013, according to “Mobile Users and Usage: It’s Personal,” a new study by eMarketer.

“With mobile usage now pervasive, eMarketer believes that mobile will develop into a ubiquitous platform for messaging, social networking, entertainment and Web access,” eMarketer Senior Analyst Noah Elkin said.

The study found the mobile subscriber population is not uniform. According to ComScore, 53 percent of mobile phone subscribers are women, while 47 percent are men. However, there are more male subscribers in the 18 to 44-year-old age range. There also are variations in usage by age, income, ethnicity and device type.

Text messaging has surpassed voice calling as the most popular means of communication on mobile devices. Nielsen found that during Q1, the average American mobile phone subscriber sent or received 486 text messages per month, but only made 182 calls.

During that same time, those 13 to 17-years old sent and received more than three times as many texts on a monthly basis as those 18 to 24-years old, and more than 50 times as many texts as those 55 to 64-years old.

In addition to text messaging, mobile Internet usage also has been steadily increasing. eMarketer found this growth will continue during the next five years, with the number of mobile users accessing the Internet to rise from 73.7 million this year to 134.3 million in 2013.

The mobile Internet user population is about one-third the size of the population that uses wired Internet, and smartphones have helped to bridge the gap between mobile Web and desktops.

“Growing sophistication in users, devices and usage patterns will mean increased opportunities for marketers to connect with consumers, particularly among the growing population of smartphone users,” Elkin said. “Yet marketers must take seriously the highly personal relationship users have with their mobile devices, and respect the need for a value exchange.”