GDP with PPP compensated is on par or better in NW Europe and for all of the EU it's about 70% of the USA (which has about 100m more people than the USA). You'd expect it to be a lot closer.

Yes it is most likely to be less but not a factor of 3 simply based on purchasing power alone.

The US also has close to no data privacy protection, so you can also practice whatever targeting and manipulative nonsense you want on your subjects.

The delta there shows what that's worth.

Do you mean US advertisers are paying three times more to FB than the advertisers in the EU?

Please don't confuse things. FB users aren't FB's customers. FB doesn't get any money directly from the (on average just a little bit) richer US users.