Ahead of Facebook's initial public offering this month, brands are piping up about the value of advertising opportunities on the social network. Units such as Facebook's "Sponsored Stories" run without the tracking cookies attached to Yahoo! and Google ad units, and have to be evaluated on the "impressions" model, making some brand reps such as Kia's vice president of marketing, Michael Sprague, question the value. WPP's Martin Sorrell added fuel to the fire when he acknowledged that clients "are increasingly starting to look at the value of those [social media] investments." Facebook has responded by starting ad-measurement initiatives with Nielsen and comScore.