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Adblock Plus, the world’s most popular ad-blocking tool, today announced it has passed 100 million active users. Just four months ago, the tool had passed 500 million downloads, which shows a decent retention rate, given that Adblock Plus has been around for 10 years and many have naturally downloaded it more than once.

The company used to highlight active users, and the latest number we could find was, indeed, 50 million. But ever since Adblock Plus updated its privacy policy, we have been told over and over again that it is simply impossible to accurately figure out how much its user base has grown.

“These are our latest numbers, which we’ve been working for months on getting more accurate without breaking our strict privacy policy,” Adblock Plus head of operations Ben Williams said in a statement. “Harder than it sounds … Happily, we’ve got some stupid smart data scientists who’ve figured out a way to get an accurate estimation.”

The company didn’t explain how exactly it got to this figure. But there is one large caveat: If you use Adblock Plus on multiple devices, you will be counted as more than one “user.” The figure represents “active installations,” as Adblock Plus has no way of tracking users across computers, tablets, and phones.

Still, with 100 million active installations across the desktop (Firefox, Chrome, Opera, Safari, IE) and mobile (Android and iOS), Adblock Plus has a massive reach that the advertising industry can’t ignore. Eyeo, the Germany-based company behind the Adblock Plus software, is trying to leverage these users to change how content is monetized online.

The company has an Acceptable Ads initiative, which selectively blocks advertising. Non-intrusive ads (the whitelist now has over 700 entities) are allowed through in an attempt to find a compromise between users and advertisers.

And earlier this month, Adblock Plus teamed up with Flattr to unveil a new project that is meant to skip ads altogether. Called Flattr Plus, it aims to let users automatically donate money to websites they use.

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