-By Mike Shields

Mobile Internet usage is growing exponentially—undoubtedly boosted in large part by smartphones like the iPhone. However, despite the hype surrounding the segment, just a quarter of wireless subscribers logged onto the Web via their mobile devices in July.

According to a new report issued by Nielsen, there were 56.9 million mobile Web users in July of this year. That represents a healthy 34 percent spike in audience versus the 42.5 million mobile Web visitors tracked last year.

However, per a separate Nielsen report issued in January—there are close to 225 million total mobile subscribers in the U.S.; therefore, mobile Internet penetration is roughly 25 percent, still trailing PC Web usage and lagging far behind mainstream media like TV.

As for which demographics are fueling the increase in mobile Web penetration, Nielsen’s report offers some surprises.  Despite the stereotype of young urban adults and business travelers driving the purchase of so many iPhones and Blackberry devices, it’s teens and seniors who are coming onto the mobile Internet in droves. Nielsen recorded a 45 percent surge in usage among teens 13-17 in July, as well as a 67 percent spike among users 65 and older.

Overall, the mobile Internet still skews male (53 percent), per Nielsen. But just as the female auditence started gravitating to the male-dominated Internet around a decade ago, women are increasingly using mobile devices to surf the Web. According to Nielsen, in July the female audience soared by 43 percent, versus a 26 percent growth spike seen among men.

And while men tend to go online via their mobile devices for technology news, sports and games, women are more inclined to use mobile for social networking and shopping. Among the top male skewing mobile sites in July ranked by Nielsen are NBA.com, NFL.com, IGN.com and CNET.com. Meanwhile, for women the top mobile destinations included Target.com, MySpace.com, Facebook.com and Kraft.com.