According to a recent study, Apple mobile iOS devices are presenting a huge blindspot to advertisers who utilize tracking software. These devices are used by 130 million people, but up to 80% running the Safari browser don't count paid-search conversions (clicks) because cookie tracking is blocked. Overall, when you count all browsers, an average 38% of all paid-search clicks are not being counted.
These numbers are relevant for third-party cookies, not first-party cookies which come only from websites you visit. Because you don't get paid for what can't be measured, this blindspot poses a growing challenge to the online advertising industry, and especially to Google. The way that advertisers can overcome this problem is to use first-party cookies served from the Websites people visit, or to come up with better ways to measure the performance of online ads.