Google has removed all ad blocking apps from Google Play, its online store for Android applications, on the basis that they interfere with other services. Searches for "adblock" and related terms on Wednesday evening no longer returned any references to ad blocking software.

Till Faida, co-founder of Adblock Plus, condemned the decision, calling it a threat to consumer choice. "By unilaterally removing these apps, Google is stepping all over the checks and balances that make the Internet democratic," he said in a statement, suggesting that Google's actions threaten Internet freedom in the same way that SOPA and PIPA did.

Google did not immediately respond to a request for comment. In its letter to Adblock Plus, Google said the software violates Section 4.4 of the company's Developer Distribution Agreement.

Section 4.4 states, "You agree that you will not engage in any activity with the Market, including the development or distribution of Products, that interferes with, disrupts, damages, or accesses in an unauthorized manner the devices, servers, networks, or other properties or services of any third party including, but not limited to, Android users, Google or any mobile network operator."

[ Read Android Chief Andy Rubin Steps Aside. ]

By enforcing these terms, Google is following a path blazed by Apple, which takes an active role in limiting the availability of certain kinds of applications.

While ad blocking software may not be available in Google Play or in the iTunes App Store, iOS users can download third-party browsers like the Atomic that come with built-in ad blocking support. It remains to be seen whether Google will tolerate ad blocking embedded in third-party browser apps.

Android users can still download Adblock Plus directly from the company's website.

Google's recent actions suggest growing concern about ad blocking. In February, the company made changes to Android that required Adblock Plus users on Android devices to manually configure a proxy to use the software.

Three years ago, only a very small percentage of Internet users chose to block ads. In 2009, about 5% of users of Mozilla Firefox used Adblock Plus, and the percentage was significantly less among users of other browsers. At the time, Adblock Plus had been downloaded 49 million times.

Today, Adblock Plus has been downloaded more than 200 million times. It's available for Mozilla Firefox, Google Chrome, Android and Opera and it's but one of several popular ad blocking extensions available. Ad blocking rates now average an estimated 10%, according to Block Metrics, a company that tracks ad blocking, and have surpassed 50% for some websites.

At one online gaming news website, Destructoid, somwhere between 42-46% of its online ads were being blocked over the course several days recently. In a phone interview, founder and publisher Niero Gonzalez said he has heard from many other publishers who reported similar figures. "I was shocked," he said, noting that the lowest ad blocking rate reported in these discussions was around 36%.

Ad blocking is most popular among sophisticated Internet users, who appear to prefer Google Chrome and Mozilla Firefox, he said. Among users of Internet Explorer, he said, only about 4% block ads on his website.

For Gonzalez and other online publishers, this represents an existential crisis: Most websites need ad revenue to survive.

Using Block Metrics, the service that alerted Gonzalez to the extent of ad blocking on his website, Destructoid implemented an overlay message that was displayed to users of ad blocking software (detection is not foolproof). It appealed to readers, asking them not to block the website's ads.

But the message didn't go over very well and the ad blocking continues. Gonzalez claims that disinterest in the appeal came despite Destructoid's adherence to best practices, to prevent obnoxious ads from being shown.

Neil O'Connor, CEO of Block Metrics, said in an email that the percentage of Internet users with ad blocking software varies, but is particularly high for gaming websites.

"The 40-45% reported by Destructoid is a high block rate, although we've observed higher," he said in an email. "Game sites are hardest hit with an average of 30% of their ads blocked (minimum 8.5%, maximum 61%)."

"The news is better for other site genres, where we're currently observing an average 17% block rate (minimum 2%, maximum 52%)," he continued, noting that most websites currently using Block Metrics have technical audiences and as a result more of their ads are blocked. "As a guideline to websites in general, we would suggest they have an adblock rate of approximately 10%," he said.

However, O'Connor says usage of ad blocking software is growing. "We're currently gathering historical data but provisionally estimate yearly growth of at least 20%," he said.

O'Connor says advertising isn't going anywhere but he sees the need for alternatives. "Users will demand it and the industry needs to innovate," he said, adding that Block Metrics intends to provide an alternative by giving users the option to support websites without viewing ads. Presently, the company provides a way to ask those with ad blocking software to reconsider.

However, technical approaches to detecting ad blocking software are unreliable. "There are constantly some scripts popping up that claim to be a solution against Adblock Plus," said Faida in an email. "However, browser extensions can modify the content of a page before the website is loaded, that's why the community of filter list authors will always be able to find a solution pretty quickly. This is more like a cat-and-mouse game which is a waste of time for everybody. We'd like to encourage websites to listen to what their users are telling them if they block their ads."

And website visitors do want to be heard. Gonzalez heard from a critic of this Block Metrics solicitation and went to the trouble of corresponding with the reader. Through an earnest personal appeal, he managed to convince the reader to disable ad blocking on Destructoid. But the experience left Gonzalez feeling he had won a small victory amid a war that has already been lost.

"If left unchecked, small publishers like me may face an ad block rate of 75% or higher with no way to pay my bills," Gonzalez wrote in a blog post. "I'm not going to lobby congress to make ad blocking illegal. That ad-block percentage is not really negotiable, and is only going to go up. Other technology trends are also closing in on independent publishers: ad rates are dropping, mobile adoption is booming and dragging old ad models that don't work into them, and (let's be honest) my Internet generation expects everything to be free, cheap, and plentiful."

Gonzalez concludes his post on an optimistic note, promising that Destructoid will make it one way or another. But it's clear that the Internet publishing model faces challenges, both for smaller, independent websites and more prominent media operations. When The Atlantic solicits content without paying for it, it's hard not to conclude that times are tough all over.

"Publishers need to have a frank conversation with their readers," said Gonzalez.

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