I would expect that it contributes to sales both by creating a fake sense of validation from others and by creating a fake sense of urgency (get it while in stock!), both of which are good tools to compromise decision making of the buyer.
Crowd selection isn't negative, in fact without it we d just be picking fruit at random in the forest with a good chance of dying of poisonning, instead of chatting online... on a popular forum we think like minded people also frequent, having opinions we suppose we'll be interested in on posts that are ranked by upvotes :p
Just because everyone believes the world is flat doesn't make it so.
At least in school in France we now learn not to blame ONLY the little guy voting out of despair, but also the unfair winners of WWI. Without that crowd deciding together to do something, anything, where would Germany be today ? Hard to be optimistic in 1933. And it drove the WWII winners into building a better post-war reparation program, not only explained by the communist scarecrow but also the lessons of the inception of WWII in Germany: revenge against abject humiliation.