I'm pretty sure when we talk about "transfers" here, we aren't talking about "user in US requests data from URL that is served from server in EU". We're talking about user-data from/about/on EU citizens that is stored on servers in the US (or transferred there from servers in the EU with the intent to keep it on servers in the US). The former is fine[0], but the latter is not.

[0] Obviously if there is some web forum hosted in Germany that a bunch of Germans living in Germany post to, and I -- in the US -- visit the forum, that involves data leaving Germany and flowing to the US. There's nothing wrong with that.

Very simple example - I'm in EU, and I like and post a comment on a photo of Rihana. How do you do that on the backend, without transferring data or keeping all the data in EU?

Even if you keep data for individual profile in corresponding country, any interaction with a content outside of EU is impossible without data transfer.

When Facebook's size, db backends are sharded. When someone looks at a page, any page, queries are hitting a number of sharded backends.