Facebook’s ad targeting algorithms are about to get a new firehose of valuable and controversial personal data.

Starting next month, the millions of Facebook “Like” and “Share” buttons that publishers have added to their pages and mobile apps will start funneling data on people’s Web browsing habits into the company’s ad targeting systems. After the change, the types of sites you visit could be used to tune ads shown to you inside Facebook’s social networking service, its photo-sharing service Instagram, and mobile apps that use Facebook’s ad network.

Facebook first offered the Like button to publishers in 2010 as a way to help people tell friends and the company what was interesting (see “10 Breakthrough Technologies 2011: Social Indexing”). The buttons take the form of a snippet of code to be added to a page. That code directs a person’s browser to contact Facebook’s servers, allowing them to know the page you’re visiting, and to see the “cookie” files that Facebook pushes to its users’ browsers to identify them.

The fact that Facebook offers to track people’s Web browsing has long concerned privacy campaign groups. Not long after the Like button’s launch in 2010, the Electronic Frontier Foundation and other organizations wrote an open letter to Facebook CEO Mark Zuckerberg that asked him to set the buttons to only collect data if someone clicked on one.

Facebook continued to let its buttons log data, and finally said in 2014 that it would “soon” use it to target ads. The company said in a blog post Tuesday that it will now officially start. The post also announced a new privacy setting that lets you opt out of seeing ads targeted based on data collected on your online activity.

Rainey Reitman, activism director at the EFF, says that is not enough, because anytime you load a page with a “Like” or “Share” button embedded, Facebook will still know about it. “Promising not to use information is not the same as promising to actually delete the data,” she says. “The ‘Like’ data is especially problematic. Most people probably don’t even realize that whenever they load a page with a ‘Like’ button on it, Facebook gets a little information on them.” Facebook did not respond to a request for comment by time of publication.

Reitman says it would be better to design the buttons to send data to Facebook only when someone actively engaged with them. In place of its new privacy option, the company should instead agree to respect the “do not track” standard under development, she says. It allows you to change a setting in your browser that signals to publishers that you do not wish to be tracked across different sites. The exact implementation of the feature is still being worked out, but one version of it would have sites stop data collection altogether if a person had turned the do not track setting on.