As to the effectiveness of retargeting / remarketing, you may find this article from VentureBeat useful:
Over 90 percent of marketers say that retargeted ads are as good as or better than the gold standard in digital marketing, search ads, according to a recent survey by leading retargeter AdRoll.
And don't forget, you can combine search and retargeting, by using retargeted search advertising. That's when you search on, say, Converse Chuck Taylors and the next thing you know every time you sign on to Facebook you're seeing ads for Chucks in the right-hand column...
- yes, retargeting is very