Regardless of the taxing system, Facebook would already show you as many ads as possible. It makes no sense to me that they would show more ads in EU in that case, because that means that they could also increase the number of ads shown in the US and increase their profit. If anything, it would just cut their surplus.
I don't believe that's entirely true, Facebook has endless channels (e.g. messenger) in which they could introduce or increase their advertising frequency (think Forbes). They could even disclose to end-users that these additional advertisements were being served specifically as they are within the EU, similar to the ubiquitous cookie pop-ups they are already well adjusted to seeing.