re: Twitter co-founder: "The Internet Is Broken."

I have old man questions that have always confused me. Does Twitter only make money from ads it sells to vendors? If so, is the money paid up front or is it on # of hits on an ad? Combination of both?

I ask this because to the best of my knowledge I can't ever recall hitting a link for an ad while on an internet site. Other than when executing a search to purchase an item.