Facebook's plan to stop ad blockers has already been foiled. Adblock Plus has found a way to strip ads from Facebook, even when they're served up in Facebook's new ad blocker-proof format. Anyone with a fully updated version of Adblock Plus should once again be able to avoid ads in Facebook's sidebar and News Feed.

The method may be flawed, however: Facebook says that in its attempt to remove ads, Adblock Plus is also removing regular posts. "We're disappointed that ad blocking companies are punishing people on Facebook as these new attempts don't just block ads but also posts from friends and Pages," a Facebook spokesperson said. "This isn't a good experience for people and we plan to address the issue."

Imperfect as it may be, the release of a new ad blocker is still bad news for Facebook, which rolled out its supposedly unblockable ads just two days ago. It's entirely possible that Facebook has additional measures it can take to subvert ad blockers or that it expected that this type of back-and-forth battle will ensue. But the fact that Adblock Plus responded in two days' time doesn't bode well; Facebook wants to display ads, but it's hard to imagine it wants to devote resources to outsmarting a browser extension every 48 hours.

"This is a cat-and-mouse game; so their next circumvention might come at any time."

Adblock Plus is already being clear that it's ready for battle. "This sort of back-and-forth battle between the open-source ad-blocking community and circumventers has been going on since ad blocking was invented; so it’s very possible that Facebook will write some code that will render the filter useless — at any time," Adblock Plus' Ben Williams writes in a blog post. "If that happens, the ad-blocking community will likely find another workaround, then Facebook might circumvent again, etc."

To circumvent ad blockers in the first place, Facebook removed code that explicitly identified ads, making them appear more like regular Facebook posts (it was a behind-the-scenes change; users still saw a "sponsored" disclosure). But apparently it didn't go far enough. Williams tells The Verge that beating the system again "was just a matter of finding the non-standard indicators they began using" and then filtering them out. But he added, "I would stress, though, that this is a cat-and-mouse game; so their next circumvention might come at any time."

Facebook said on Tuesday that it felt comfortable circumventing ad blockers because its ads weren't annoying and it was giving users increased control over what types of ads they see. Behind all of that, of course, is the fact that Facebook can't exist without ads, which are far and away its primary source of revenue. It's a pretty reasonable argument, given that Facebook is a free and optional service. But some people using ad blockers say that they're necessary to maintain their privacy online, since ads, including Facebook's, can track your interests around the web.

"Ad blockers are a blunt instrument," Facebook's spokesperson said, "which is why we've instead focused on building tools like ad preferences to put control in people's hands."

Update August 11th, 1:10PM ET: This story has been updated to include comment from Facebook.