It has control as Germany is part of the Eurozone, the ECB is subjected to the EU treaty, operating under its guidelines, and is accountable to the European Parliament, in which the people of Germany have a say. Maybe it is not as accountable as we wish it were, but it is far from what Facebook would be.

Isn't the Bank of England privately owned? Or perhaps it merely was privately owned. If Facebook does this, they had better be prepared to be nationalised.

Is “a vote in” and “control” really the same thing?

In case of Germany it’s very close to be the same thing.

EU has many checks and balances, is governed by Germany and other like-minded countries, and has given lots of guarantees as well as an option to leave. If Facebook decides to discuss on that level, maybe an actual agreement could come out of that.

Facebook can come and expect to discuss on the level of trust that a country gets when it has existed for less than 2 decades and has been treating its citizens/users like an authoritarian dictatorship surveillance state. There are a number of other parallels, and none of them put FB in a good light.

It's not going to be a very amicable discussion.

If the EU goes tits up, Germany will be using marks again before the month is out. The EU could never have been started if member states had to give full control of monetary policy to the EU Council.