I can be confident because Facebook spends more time and money studying user interaction with their advertisements than any other single aspect of their business. They have focus groups, they study eye movement with iris tracking, and they are leading authorities in online marketing. They don't make a change to their online ads without understanding 100% of the variables.

And, for the record, this analysis of Facebook's investment in user study isn't mere conjecture. I've been lucky enough to attend a talk by Facebook employees on their efforts.