More than 18.3 million unique U.S. smartphone users turned to their devices for social networking access in July 2009, up from 6.4 million users a year earlier, according to new research issued by Nielsen.

Social networking now accounts for 32 percent of all smartphone activity, Nielsen notes, adding that Facebook leads the pack with 14.7 million smartphone users – MySpace follows with 7.1 million, with Twitter at 4.1 million.

“Social networking is probably the most important thing going on with mobile devices because the devices lend themselves to more social networking activities,” said Nielsen’s research director for technology and search Jon Stewart. “When you’re sitting at your PC you’re not going to necessarily be Twittering about sitting at your desk, but when you’re out and about in the world, you’re more likely to have content and experiences that you want to share.”