More and more, online services are making their best to provide free services in exchange personal data [trust, and habits].
The currency used to pay this services—like Facebook, Twitter, or Google—is private data, which turns out to be, pretty often, really sensitive information.
Everytime you click an ad on Google, a hashtag on Twitter, or content on Facebook, you can expect to be bombarded with similar content on the future, with related products to buy, or with brands that advertise specifically on 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 when we see this kinds of things that we realize we are actually being tracked with a purpose. Have you ever regretted a click? I have.