Facebook’s willingness to work with third-party developers and pull in third-party content, and its encouragement of content-sharing between members has helped the social network’s population surge to more than 175 million members. That openness is also boosting Facebook’s status as a traffic-driver: the social net has topped Google (NSDQ: GOOG) as the number-one source of traffic to a number of large sites, including PerezHilton.com, CafeMom.com and events site Evite.

And it’s a trend to watch, since, as AdAge notes, Facebook now only gets about a third of Google.com’s unique visitors, per comScore, and the traffic—both the clicks and the eyeballs—is what generates search revenues. Companies spent over $12 billion on search marketing last year.

Much of the Facebook-driven traffic comes from links that members post via areas like “Notes” and photos. If Facebook’s influence as a traffic source continues to rise, the next step would be to figure out how to monetize the traffic to those areas with paid search. That would be one way to entice Microsoft (NSDQ: MSFT) to renew its search deal (and give Microsoft a better return on its $240 million investment in the social net).

Photo Credit: pshab

• @ CES: Norwest’s Chang: Things Could Get Scary For Facebook, Slide in ‘09