All on-line ads are mostly worthless - most clicks are from clickfarms, and the genuine parts are mostly by clickjacking or accidental misclicks. Almost no one really wants to buy stuff via ads. Ad agencies keep telling companies that advertising online will generate lots of revenue, but between all those messy, noisy metrics and people generally not understanding a thing about statistics it's hard to see what part of revenue can be really attributed to ads, and whether or not they're worth the money invested.