Imagine you have somethng to sell, I have the ability to place ads in people's homes via their electronic devices and I give you the choice of placing an ad for your product in one of two places.

You can place the ad into everyone's photo albums so that when they are flipping through photos they will see your ad.

Or you can place your ad in everyone's telephone book so when they are looking up the contact information for some source of some product, they will see your ad.

Which one would you choose?

Of course, in the real world, advertisers can choose both. They will advertise both on Google and Facebook.

And sure, it matters what the product is.

But overall which do you think is more valuable?