Buying a sponsored AdWords ad on Google results in people viewing it while they are actively looking to solve a problem or buy a product.
Buying an ad on Facebook that happens to be related to something they once had interest in, and may still do, will result in it being shown while they are not always looking to solve a problem or buy a product, but to entertain themselves, and most importantly may not care about that subject at that moment.
Online advertising is not a cut and dry field. You can get great results from either place, but you won't get the same results from both, at least in my experience.
The thing is that Google dont kneed to place ads on Google+, but they will know your personal information and place better targeted AdWords for each person.
Buying an ad on Facebook that happens to be related to something they once had interest in, and may still do, will result in it being shown while they are not always looking to solve a problem or buy a product, but to entertain themselves, and most importantly may not care about that subject at that moment.
Online advertising is not a cut and dry field. You can get great results from either place, but you won't get the same results from both, at least in my experience.