So you thought online social networking didn’t have a business model outside of the sheer genius of ‘pay another 10 bucks to network with people who’s email adresses you already have’? Oh, yee of little faith.

It turns out online networking isn’t about Six Degrees of Kevin Bacon, or other such esoteric party games. It’s about classified advertising.

Still not convinced? Imagine the Guardian buying

Well, that at least is the kind of the thing suggested by the latest news in the US. Newspaper giants Knight Ridder and The Washington Post have invested in, a social network that connects like-minded people together. The hope is that will shore up the newspaper’s traditional classified advertising model. No, really.

The new crop of online social networks such as Craig’s List (which quietly launched in London this year), Friendster, MeetUp and has lead to trusted netorks being formed amongst the most active users. This is ultimately leading them towards more reliable information on purchases and services in defined geographical areas – especially when it comes to house sales and plumbers, not to mention prospective boyfriends and girlfriends.

And it’s all getting a bit serious. Three weeks ago, San Francisco-based won $6.3 million in VC funding, while Knight Ridder and The Washington Post Co. also took an undisclosed stake.

Hilary Schneider, CEO of KnightRidder Digital was quoted as saying: “We sense that there’s this emerging market … that people use other people in order to connect with listings and clearly, a big part of our revenue is classifieds.”

The ultimate idea is to integrate self-published referrals with KnightRidder content such as movie and restaurant reviews. The Manchester Evening News, watch out., launched in July, invites people to build a variety of social and career networks, or tribes, to help them accomplish their goals. Typical users are highly educated urban dwellers aged 20 to 40, according to the company, so don’t expect to find Grandma there.

But it’s early days yet, and the key demographic is non-newspaper readers – people newspapers are understandably desperate to attract.

Just ask The Times. It, along with the Independent (which was first) recently launched tabloid versions to try and attract younger, commuting readers in London.

If these latest move in the US are anything to go by, there could come a day when we’ll see social software come free with our daily newspaper.