Jeff Smith is a diligent social-networking user, but he doesn’t own a PC.

“I prefer a cellphone and a service for a cellphone,” says Smith, 40, a postal worker in Detroit who served as an Army Ranger in Desert Storm and Somalia.

For about a year, Smith has used MocoSpace (for “mobile community space”) to chat, meet people, search the Web and play games. “Anything else feels like too much.”

The majority of people who participate on social networks do so from their PCs. Yet a growing number — many of whom can’t afford a PC or would rather not use one — are using mobile devices to tell their friends where they are and what they’re up to and for sharing pictures.

Mobile users are an important part of the mix for behemoths Facebook, Twitter and MySpace. But many folks are migrating to a new crop of mobile-only social networks such as MocoSpace, Mig33 and Peperonity. MocoSpace has emerged as a favorite in the U.S., where it is available in 22 cities, including New York, Seattle and Los Angeles. It offers chat, instant messaging, photo- and video-sharing, and games.

The number of people who use social networks from their smartphones skyrocketed 187%, to 18.3 million unique users, in July, compared with the same month a year earlier, says Nielsen. Social networking is among the fastest-growing activities on mobile devices, along with search and checking news, says Jon Stewart, Nielsen’s research director for technology and search.

With so many eyeballs increasingly fixated on mobile devices, opportunities for advertisers abound. Visiongain Research predicts mobile-social-network-related revenue will reach about $60 billion in 2012.

For the BSO-chasing crowd (that’s Bright Shiny Object in social-snakeoil-salesman-speak), mobile will finally have its 15 minutes of fame in 2010, only to be immediately eclipsed by wi-fi ubiquity by 2012.