We cover a lot of day-to-day efforts by retailers to reach Facebook users, now Hitwise has a little more data on how well they’re doing. “Social networking and Forums” is becoming a more significant type of traffic source for the top 500 US retailer’s web sites. This past December, the broadly-defined category drove 37% more traffic than a year prior and made up 13% of overall traffic sources.

Hitwise doesn’t clearly say what portion of the category is made up of Facebook, but it does offer some more data on that front. “During the holidays over 2% of the traffic to Facebook (the 2nd ranked website in the US) visited a website in the Retail 500 immediately after,” Hitwise research director Heather Dougherty writes today.

During the start of the holiday shopping season at the end of November, upstream traffic from Facebook notably went above 3%. Then it gradually drifted down, presumably as more people bought presents. Facebook had at least 108 million US users in December, depending on which numbers you believe, so it’s likely that a few million people visited retailer sites during the month.

Still, most retailers are still experimenting with a variety of approaches to building a presence inside Facebook. Many companies are not trying to send users from Facebook to their sites, and are instead using Pages, Events, advertising and other site features to reach users and directly sell goods to them. Hitwise isn’t measuring all of this retail activity happening on the site.