The biggest problem in online advertising is proving that ads can spur sales. Facebook (FB) is moving a step closer to that goal by following people to the grocery store.

No, Mark Zuckerberg's minions aren't stalking you in the dairy section. Instead, Facebook is getting mountains of data from stores about the purchases its users make. It's trying to make the connection between its advertising and what users buy.
It wants to show advertisers that it showed Joe Consumer an ad for Folgers and within days he bought a can of Folgers. If it can pull that off, well, its ad prices will go through the roof.

Here's how it works:

Facebook has begun paying a company called Datalogix, which has the purchasing information from about 70 million American homes, the Financial Times reports. Datalogic gets much of that data from loyalty cards at grocers, drugstores and other retailers.

Facebook buys that information and, using its dark magic, matches it with specific users. It relies on email addresses and other information people submit when they open a Facebook account. From there, it's very easy for Facebook to connect the dots. It shows some users an ad for yogurt, for example, and waits to see whether those people buy that brand of yogurt.

The two companies say they have measured 45 campaigns, the Times reports. And 70% of the time, every dollar an advertiser spends on Facebook gives it $3 back in incremental sales.

How obnoxious is this? There really is no privacy when it comes to Facebook. And users can't go on Facebook and opt out, the Times reports. Instead, they have to go to the Datalogix website to opt out.

Facebook and Datalogix say they anonymize the data and don't share information about individual users' purchases with advertisers. But privacy advocates tell the Times that Facebook shouldn't be doing this without express consumer approval.

Over at Marketing Land, Greg Sterling says tracking offline sales has always been the holy grail of online measurement. There have been a couple of privacy bills related to consumer data, but they have stalled in Congress, he adds. The Obama administration wants to push tougher rules to Internet companies, but even those rules would be voluntary.

The only thing that may get those rules going is a consumer revolt. But what's it going to take for people to finally be fed up? I can see a future in which we buy milk at the grocery store and Facebook sends an ad to our phones saying: "You really need Oreos to go with that milk you just bought. They're in Aisle 5 for $2.49."

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