Politicians in Washington are fond of ripping into Facebook for its privacy practices. The latest round of excoriation came last week after the company revealed its ambitious plan to create a worldwide cryptocurrency called Libra. “Facebook is already too big and too powerful,” tweeted US senator Sherrod Brown (D-Ohio), “and it has used that power to exploit users’ data without protecting their privacy.”

Senator Josh Hawley (R-Missouri) told Yahoo Finance that he wouldn’t trust Facebook with his money. “I don’t trust Facebook with anything,” he said.

Just one problem: Despite their professed concerns with Facebook, both senators' campaign websites—sherrodbrown.com and joshhawley.com—have an invisible piece of Facebook technology, called a pixel, that tracks when anyone visits their homepages and shares this information with Facebook. Hawley’s website even shares when visitors donate and the exact donation amount. Facebook can then associate that information with an individual’s Facebook account.

Brown’s office did not respond to a request for comment, and Hawley’s office, when reached by WIRED, did not respond to questions.

Brown and Hawley are hardly alone in sharing their website visitors’ data with Facebook in this manner. Over the past two months, I surveyed the official campaign websites of 535 US politicians. As of June 14, 81 sitting US senators, including Brown and Hawley, have Facebook tracking pixels embedded somewhere on their campaign websites; 31 of them send exact donation amounts. As of last Friday, at least 176 members of the House of Representatives also have the Facebook pixel on their campaign homepages. And almost every 2020 presidential candidate uses this kind of tracker, too, including President Donald Trump.

Hamdan Azhar is a data scientist and journalist based in New York.

And this should be underlined: Facebook’s pixel technology, which is meant to help target Facebook ads to visitors, must be approved by websites on which it operates. These politicians—or at least their campaigns—have actively signed up to allow Facebook to track their visitors.

Why does this matter? Politicians of both parties today claim to care about our privacy, and many of them have positioned themselves as ardent critics of the intrusive role of big tech companies in our lives. Yet, when it comes to their own campaigns, a significant number of them are sharing our web browsing data with Facebook, using computer code buried on their websites. With Facebook having become a core part of the modern political campaign apparatus, can we really trust politicians when they claim that they will be the ones to defend our privacy and protect us from Facebook?

How Facebook Tracks You Across the Web

In recent years, public concern around privacy and Facebook has focused on the data we willingly share with Facebook on its social media platforms—status updates, photos, comments, likes—and how Facebook in turn shares that data with third parties. But Facebook’s tracking of our online behavior is far more widespread, extending to millions of other websites.

One way Facebook does this is with the Facebook pixel, a piece of computer code that a website owner embeds into their site. Whenever you load a website containing a pixel, the pixel immediately sends information back to Facebook, including date, time, URL, and browser type. Facebook can match that data with your Facebook profile.

But a pixel can capture much more information, depending on how it is configured by the website owner, like logging when a visitor adds items to a shopping cart or searches for something. Website owners don’t have access to the raw data generated by the pixel, which is sent directly to Facebook’s servers for temporary storage and processing, but they can use it to run more targeted Facebook ads and measure their effectiveness. For example, an ecommerce company might want to show ads to people who visited its site and left without completing a purchase (this is called retargeting). Or a nonprofit might want to solicit donations from non-donors who have behavioral and demographic characteristics similar to those who have previously donated (using Facebook Lookalike Audiences). Or an elected official might want to measure whether people who see their Facebook ads are more likely to make a campaign donation on their site.

What We Found 81 US senators use Facebook pixels somewhere on their campaign site.

31 senators share exact donation amounts with Facebook.

176 US representatives have Facebook pixels on their campaign homepage.

Nearly all 2020 presidential candidates uses Facebook pixels on their campaign sites.

Facebook rolled out its pixel feature to all advertisers in 2013. I first became familiar with pixels—and with digital advertising in general—while working at GraphScience, an advertising technology startup. In 2014, I joined Facebook as a data scientist working on advertising research, a position I held until 2016, when I left to work at a blockchain technology startup. But you don’t need to work at Facebook to study pixels. My analysis here is based solely on publicly available data, and in reporting this piece, I used the publicly available Facebook Pixel Helper Chrome extension to inspect websites for Facebook pixels.