Mobile social network MocoSpace is crowing that it now has three million registered users. The news follows on the heels of the Boston-based start-up being ranked as the third most-trafficked mobile site in the US after MySpace and Google, according to stats from mobile browser company Opera Software.

Not bad for a company that, according to co-founder Jamie Hall, has grown largely by word-of-mouth.

One of the first made-for-mobile social communities to start life as an off-deck service, MocoSpace has a different feel than most other social networks, something Hall believes works to its advantage. MocoSpace users flock to the site to keep in touch with their friends wherever they are. The major emphasis is on communication, and it offers plenty of ways to communicate - IM, chat, e-mail, messaging, blogging, e-forums and even e-cards.

MocoSpace is a totally open community whose users tend to skew younger (ages 17-23) and urban. They're also quite active on the site, generating more than 1.5 billion page views a month with the average member visiting the site more than once a day.

Hall told us that the company has no plans to jump into the location-based services craze, something he believes works much better in more "closed" communities. It seems to us that if three million users are generating 1.5 billion page views every month on the site, then they're probably telling their friends where to find them anyway, without needing the help of a GPS locator.