Mobile operators face a new battle for control of the customer relationship. But this time it's not about ringtones, games and search; it's about what is quite possibly their most valuable asset: the application you use to make a phone call.

The challengers are the online social networks — with their growing reach, power and innovation — and their legions of developers. When online social network meets mobile phone, the frontline is the humble address book.

Worldwide, nearly 600 million people visited an online social network in June 2008, according to comScore. The leading social-networking sites — Bebo, Facebook, Friendster, hi5, MySpace and orkut — added 88 million new visitors in the first half of the year. And people keep coming back: 96% of online tweens and teens log onto a social network at least weekly.

Now online social networks are moving from the desktop and laptop to the cell phone, with versions of Facebook and MySpace for the BlackBerry, iPhone and mobile Web, as well as a new group of “social-network aggregators” such as Yahoo!'s oneConnect. In less than a year, more than five million of Facebook’s 100+ million users have mobilized.

There is a larger, more ubiquitous and far more meaningful social network than Facebook, hi5 or MySpace — or all of the online social networks combined; it’s the network of people that you, I and everybody else actually talk to (and get together with) each day. It’s our real social network, and includes people, businesses and organizations not part of any online community. Mobile operators are smack in the middle of this real network.