Sentence:  The dirty secret of the internet is that all this distraction and interruption is immensely profitable. Web companies like to boast about...and so on. But the real way to build a successful online business is to be better than your rivals at undermining people's control of their own attention. Partly, this is a result of how online advertising has traditionally worked: advertisers pay for <WSD>clicks</WSD>, and a <WSD>click</WSD> is a <WSD>click</WSD>, however it's obtained. 
Meaning of the word marked with <WSD> tags:  (by extension) A single instance of content on the Internet being accessed.