Companies aren't blindly paying FB billions of dollars for ads without verifying they actually deliver a return... Clearly it's working, that's the reason so many companies pay for them.

Sure, they could probably get better at managing "click farming, bots, etc", but despite these things, advertising on FB has proven incredibly valuable and there's no clear indicator that'll change soon. If anything it'll just grow more valuable across a larger user base, hence their lofty P/E ratio (implying growth).