Facebook announced on Tuesday that it will allow advertisers to purchase Sponsored Stories in users’ News Feeds based on other websites that the user has visited. This is a momentous step forward in Facebook’s plans to monetize the vast trove of data it has on its users; until this point, the site has only allowed marketers to target advertising based on the information on a user’s profile that they have voluntarily given to the site.

And it’s no wonder the marketers were hungry for advertising space in the News Feed; the response rate for ads in the News Feed are reportedly 10 to 50 times higher than those of ads featured in Facebook’s right-hand column. “Getting in the news feed is like getting on anyone’s personalized NYT front page,” says TellApart CEO Josh McFarland. TellApart is one of three companies bidding for ad space in the trial “alpha” version of the new feature.

The addition of browsing history to the knowledge that advertisers already possess about Facebook users can’t be understated. For the first time, companies advertising through Facebook can gain an almost complete grasp of users’ tastes and preferences. Though the new feature was just announced, it’s a safe bet that privacy watchdog groups will have something to say about this new ad feature. Should Facebook allow advertisers this kind of access to its users? How far do you think marketers should be allowed to go on Facebook?