Facebook is trying prove that its ads actually work by teaming up with data company Datalogix to find out how many people buy products in stores after seeing an ad on the ubiquitous social network. Datalogix owns purchasing data, drawn from membership programs at over a thousand stores, on over 70 million households in America. According to the Financial Times, Facebook is working with Datalogix to cross reference this purchasing data with its users' Facebook account information to determine how effective the site's ads are. The move is the result of increased pressure from marketers (and likely investors) since the company went public four months ago.

Facebook's head of measurement and insights Brad Smallwood told the Financial Times that so far, the study has found that for ever dollar spent on a Facebook ad campaign, advertisers earned an additional $3 in sales. Despite finding that Facebook's ad campaigns have been relatively successful thus far, the partnership with Datalogix raises a slew of privacy issues for a service that has oft been questioned in the past. While the network's previous privacy issues stemmed from the use of online data, this could be the first time the company faces backlash for using data pertaining to its users' offline lives. Facebook users have no easy way to opt out of the study — there's no option within Facebook itself — but have to visit Datalogix's site directly.

Update: A Facebook spokesperson reached out to us with the following statement about the company's new partnership with Datalogix:

We are working with Datalogix to help advertisers understand how well their Facebook ads are working. We also do this through our partnerships with companies like Nielsen and comScore and through our own advertising tool. We know that people share a lot of information on Facebook, and we have taken great care to make sure that we measure the effectiveness of Facebook ads without compromising the commitments we have made on privacy. We don't sell people's personal information, and individual user data is not shared between Facebook, Datalogix or advertisers.