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[This is a joint post with Grant Storey, a Princeton undergraduate who is working with me on a tool to help users understand Facebook’s targeted advertising.]

Facebook announced two days ago that it would make its ads indistinguishable from regular posts, and hence impossible to block. But within hours, the developers of Adblock Plus released an update which enabled the tool to continue blocking Facebook ads. The ball is now back in Facebook’s court. So far, all it’s done is issue a rather petulant statement. The burning question is this: can Facebook really make ads indistinguishable from content? Who ultimately has the upper hand in the ad blocking wars?

There are two reasons — one technical, one legal — why we don’t think Facebook will succeed in making its ads unblockable, if a user really wants to block them.

The technical reason is that the web is an open platform. When you visit facebook.com, Facebook’s server sends your browser the page content along with instructions on how to render them on the screen, but it is entirely up to your browser to follow those instructions. The browser ultimately acts on behalf of the user, and gives you — through extensions — an extraordinary degree of control over its behavior, and in particular, over what gets displayed on the screen. This is what enables the ecosystem of ad-blocking and tracker-blocking extensions to exist, along with extensions for customizing web pages in various other interesting ways.

Indeed, the change that Adblock Plus made in order to block the new, supposedly unblockable ads is just a single line in the tool’s default blocklist:

facebook.com##div[id^="substream_"] div[id^="hyperfeed_story_id_"][data-xt]

What’s happening here is that Facebook’s HTML code for ads has slight differences from the code for regular posts, so that Facebook can keep things straight for its own internal purposes. But because of the open nature of the web, Facebook is forced to expose these differences to the browser and to extensions such as Adblock Plus. The line of code above allows Adblock Plus to distinguish the two categories by exploiting those differences.

Facebook engineers could try harder to obfuscate the differences. For example, they could use non-human-readable element IDs to make it harder to figure out what’s going on, or even randomize the IDs on every page load. We’re surprised they’re not already doing this, given the grandiose announcement of the company’s intent to bypass ad blockers. But there’s a limit to what Facebook can do. Ultimately, Facebook’s human users have to be able to tell ads apart, because failure to clearly distinguish ads from regular posts would run headlong into the Federal Trade Commission’s rules against misleading advertising — rules that the commission enforces vigorously. [1, 2] And that’s the second reason why we think Facebook is barking up the wrong tree.

Facebook does allow human users to easily recognize ads: currently, ads say “Sponsored” and have a drop-down with various ad-related functions, including a link to the Ad Preferences page. And that means someone could create an ad-blocking tool that looks at exactly the information that a human user would look at. Such a tool would be mostly immune to Facebook’s attempts to make the HTML code of ads and non-ads indistinguishable. Again, the open nature of the web means that blocking tools will always have the ability to scan posts for text suggestive of ads, links to Ad Preferences pages, and other markers.

But don’t take our word for it: take our code for it instead. We’ve created a prototype tool that detects Facebook ads without relying on hidden HTML code to distinguish them. [Update: the source code is here.] The extension examines each post in the user’s news feed and marks those with the “Sponsored” link as ads. This is a simple proof of concept, but the detection method could easily be made much more robust without incurring a performance penalty. Since our tool is for demonstration purposes, it doesn’t block ads but instead marks them as shown in the image below.

All of this must be utterly obvious to the smart engineers at Facebook, so the whole “unblockable ads” PR push seems likely to be a big bluff. But why? One possibility is that it’s part of a plan to make ad blockers look like the bad guys. Hand in hand, the company seems to be making a good-faith effort to make ads more relevant and give users more control over them. Facebook also points out, correctly, that its ads don’t contain active code and aren’t delivered from third-party servers, and therefore aren’t as susceptible to malware.

Facebook does deserve kudos for trying to clean up and improve the ad experience. If there is any hope for a peaceful resolution to the ad blocking wars, it is that ads won’t be so annoying as to push people to install ad blockers, and will be actually useful at least some of the time. If anyone can pull this off, it is Facebook, with the depth of data it has about its users. But is Facebook’s move too little, too late? On most of the rest of the web, ads continue to be creepy malware-ridden performance hogs, which means people will continue to install ad blockers, and as long as it is technically feasible for ad blockers to block Facebook ads, they’re going to continue to do so. Let’s hope there’s a way out of this spiral.

[1] Obligatory disclaimer: we’re not lawyers.

[2] Facebook claims that Adblock Plus’s updates “don’t just block ads but also posts from friends and Pages”. What they’re most likely referring to that Adblock Plus blocks ads that are triggered by one of your friends Liking the advertiser’s page. But these are still ads: somebody paid for them to appear in your feed. Facebook is trying to blur the distinction in its press statement, but it can’t do that in its user interface, because that is exactly what the FTC prohibits.