If you signed up for Valve in the EU for like Left 4 Dead or Half Life or something, it would be against the law for them to share that account with Steam?

Account linking is usually done through a system that you opt into, agreeing to have that link.

This is entirely different, it's just "fuck it we've got all this data, we'll share it across and use it regardless of consent or not".

While for other things it's a completely external registration, for example I have a Steam account but also have to make a completely separate Capcom account for Street Fighter, then link the two together.

The key component is that the end user consents before any data is shared, whereas Meta's approach is just to do it regardless of consent, treating your private information as something you don't have a right to control.