More and more, online services are making their best to provide a free service in exchange of personal data [trust, habits, patterns of behavior].
The currency used to pay services such as Facebook, Twitter, or Google is private data, which turns out to be, frequently, sensitive information.
Every time you click a Google ad, a hashtag on Twitter, or a Facebook post, you can expect to get bombed with similar content in the future from brands that advertise specifically for the content you visited, or the terms you searched for.
Gently lent your computer to a friend who had to look for flights to China? Expect a lot of ads with sales to fly to China in many other pages.
It is later on, when we are visiting other sites, that we find ads related to what we were previously browsing. Have you ever regretted a click? I have.