In 2013, more than 140 million subscribers will be part of a mobile social network, and that they will generate $410 million from subscriptions alone, which doesn’t include revenues from mobile advertising, according to a report recently released by ABI Research. To get a sense of where things stand today in mobile social networks, I caught up with Justin Siegel, CEO and co-founder of MocoSpace, a free mobile social network. (The “moco” stands “mobile community,” not “mobile content,” as it does in our name). Siegel said since launching MocoSpace in the fall of 2005, the site has been on an upward trajectory, and now has 3.5 million registered users and receives more than 2 billion page views a month, mostly from the U.S. and some from Canada. To be sure, the company attracts the curious, looking for photos and profiles of strangers and friends while on the go.

Highlights from the conversation with Siegel after the jump...

“If you work in retail or construction, and occupations like that, you do not work in front of a PC. A mobile social network will be much more integral to your needs,” said Siegel, who points out that Facebook and MySpace make you register online first before you can view profile information on the phone. “It’s not about starting over [with a new social network]. It’s about starting.” Headquartered in Boston, with development offices in Israel, MocoSpace has 25 employees and has raised $7 million in two rounds of venture capital.

—History: Siegel started the company with Jamie Halls, who he has been friends since childhood. Together they previously founded JSmart, a mobile game company that was bought by the SK Group for an undisclosed amount in 2004. The two worked at the company through Skyzone, the U.S. group, before leaving to start MocoSpace.

—The opportunity: Siegel said they started the company because they thought online social networks, like Friendster, MySpace and Facebook were perfect for the mobile phone, and that soon the garden walls would come down, and mobile would be more like the open Internet, rather than a carrier deck.

—Build it and they will come: It was hard to get new users at first, Siegel admits. “Mostly it was hand-to-hand combat.” They promoted the site by going into different chat sites, social networks, blogs and bulletin boards, promoting MocoSpace for the phone. “We were fortunate that the first users we got were very enamored with the product and spread the word for us.” Today, they are primarily offdeck, and have placements with Virgin Mobile USA (and Helio, which is now owned by Virgin Mobile).

—Mobile Advertising: Currently, the market is rationalizing and the prices you may have gotten for a mobile banner once, are now falling. “It was great when you could get $40 and $50 CPMs, and some companies that drank the Kool-Aid and believed that would continue and built their forecasts on that, are going to be in trouble, but we didn’t get invited to that party, so we didn’t drink the Kool-Aid….CPMs are coming down, and most of the advertising is based on performance, or CPCs, except for fairly unique situations. They [the prices] came down as I suspected they would, and I think there is more inventory than is being sold, but that’s because they haven’t found the right price. There is interest…I’m surprised with how well we are doing with mobile advertising. I won’t go into details, but we’ll generate millions of dollars in ad revenue this year. That’s surprising to me. I’m very optimistic we can do better, but we are doing pretty darn good. We don’t even have an ad sales guy. We just use the ad networks.”

—Competitors: “We aren’t a household name, or the biggest player in the world, but we are almost by any measure, the largest pure play mobile social network in the U.S.” According to numbers provided by ComScore M:Metrics, which ranks mobile social networks by how many mobile subscribers access them, MocoSpace is hardly a leader in the U.S. It still ranks MySpace as first with 7.6 million subscribers using it on average in the three months ended June 2008. The second-largest is Facebook, which has 5.2 million subscribers.. However, in numbers provided by Opera, the mobile browser, it sees MocoSpace as a leader, ranking it as the third-largest site in the U.S. behind MySpace and Google. Siegel: To be sure, in addition to the online giants, there’s a host of other mobile social networking startups, like Loopt, buzzd and Treemo. “The mobile space has always been extremely fragmented.”

—Content Partners: MocoSpace works with Universal and Def Jam to help promote new albums that are about to be released. Recently, the site hosted a live interview with L.L. Cool J to promote a number of things he has going on, including his new album “Exit 13,” which is due Sept. 9, and a clothing that he’s selling at Sears. The interview was streamed on the Web, and on the phone, it was a text chat.