Advertisers already know about online ad metrics like cost per impression, cost per click and cost per action. But now some are considering “personal cpm.” The big winners may be celebrities and the startups that work with them.

“There is always a fear, I know, that the bubble is about to burst when a parade of actors and actresses comes through my door. This time the discussions are much more rational.” That’s what Accel Partners’ Jim Breyer said when explaining his firm’s involvement in a funding consortium that includes top Hollywood talent agency William Morris, AT&T and fellow Silicon Valley VC Venrock.

Enter social networks. The “social graph” — the data about you and your interactions with others in social networks — is a powerful new form of advertising because the social influence of individuals is amplified through social connections, as Forrester’s Charlene Li has discussed.

Essentially, social network user data can be used to measure how influential a given user is, which then determines a price that advertisers may be willing to pay for an influential user’s endorsement. Influence can be quantified through a variety of measurements, like how friends an influencer — say a star rapper — has, and how many of friends visit the star’s profile page.

A sort of “personal CPM” may emerge, as Mariam Salzman of leading advertising agency JWT describes it, according to Li. Instead of a brand buying impression-based advertising on a social networking web page, it would pay the influencer to directly promote the product.

Today, you can already see the celebrity-influencer phenomenon happening on MySpace, where musicians do everything from feature brands they endorse, to widgets of partner companies looking to spread across more profile pages.

Advertisers already pay big bucks to marketers in Hollywood, who party with stars, and try to get them to drink a certain sports drink, wear certain clothing, use a certain phone, etc. Because of the massive exposure these stars get in the press, especially in popular gossip rags, the brands themselves are seen by millions. Social networks are a relatively untapped, more measurable option.

But defining the metrics of “personal CPM” and using it to promote brands — that’s still a work in progress. It’s not clear how valuable non-celebrity influencers may prove to be, at least not yet, as commenters on Li’s piece point out. But it is clear that celebrities can already create a lot of value for brands through their influence.

It is in this context that talent agency William Morris and its Silicon Valley partners are looking to make large investments in companies based in Southern California, that can build services around their long lists of all-star entertainment clients.

During a recent event Hollywood, I talked to a number of startups that are building new services based around actors, musicians and others — and the huge number of fans that follow their every move. I came away pretty intrigued.. I’ll be covering these companies in the coming weeks.