"As long as the servers are in another country, the French government would have to create a Great Firewall to stop its citizens from accessing that content." Not really.

How to avoid Great Firewall, while blocking FB:

France could expect Facebook to deny requests for content originating in France, regardless of where FB servers are. No need for a Great Firewall; make FB implement the firewall.

Legal justification (if not ethical):

The server locations are irrelevant because the data has cross the border into France.

Enforcement (how to force FB to block itself):

Easy: make the executives criminally liable. Also, anyone working on FB infrastructure could be held criminally liable.

France is a big and important country, they have heft to get their way from companies like FB.

Geoblocking is common and if they are doing business in a country it's perfectly reasonable to expect them to do it as well as for the state taking measures to enforce non-compliance (though criminal liability of corporate executives is something we won't see anytime soon I guess).

What's not reasonable is to force them to block people in other countries (and also those taking significant efforts to conceal their identity) - at least there is no international law (compared to international deals) justifying that anyhow.

Yes, it means that facebook can't operate a company in france selling services in france and expect to ignore french law.