Earlier quoted context omitted.
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.
On top of validation and urgency that are negative, you have also a true sense of popularity that indicates that the product works, has good reputation, fits a common purpose or is priced correctly. 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…
Just because everyone believes the world is flat doesn't make it so.