At least 419 million people around the world are blocking advertising on their smartphones, according to a report PageFair released Tuesday. Most of those people -- 408 million of them -- are using ad-blocking browsers, notes the report. The use of ad blockers doubled in 2015, and the use of mobile ad-blocking browsers increased 90 percent during the same year. "The findings show that the platforms that publishers and advertisers had believed were safe from ad blocking are not necessarily safe," said Johnny Ryan, head of ecosystem at PageFair.