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: <WSD>advertisers</WSD> pay for clicks, and a click is a click, however it's obtained. 
Meaning of the word marked with <WSD> tags:  One who advertises.