Payvment, a San Francisco-based startup aiming to provide a single shopping network across Facebook, raised $1.5 million in funding from BlueRun Ventures and angel investor Dave McClure.

The company builds storefronts for retailers on Facebook so users can buy products directly on the social network, instead of having to click through to a new web site. It has built more than 20,000 storefronts since launching last November. The company wouldn’t disclose the volume of sales it was facilitating except to say that it was supporting “thousands and thousands of dollars” of sales.

Chief executive Christian Taylor said businesses lose customers when they make those customers click away from Facebook and that Payvment’s storefronts improve conversion rates and make it easier to reach new clients, since friends share products with each other. To retain customers, Payvment lets businesses offer discounts to people who “Fan” their Facebook Pages.

The other part of Payvment’s technology that Taylor says is unique is that it unifies all of its 20,000 storefronts through a single shopping cart, so if a user wants to buy sweatshirts from one store and coffee makers from another, they don’t necessarily have to check-out twice.

“Our shopping cart will jump from location to location as you go through the entire social network,” he said. “We want to be the de facto shopping network system for Facebook.” He added that Facebook’s recent announcement that it will turn external web sites into Fan Page-like experiences has expanded Payvment’s potential market reach.