Belgium would still need a way to enforce it, and unless they build their own Great Firewall, they cannot physically prevent FB from doing business in their country, nor could they collect fines from them. The best they could do would be to petition the EU and/or the US to do something about it.

They can just ask their ISPs to remove facebook.com from their DNS. That'll remove 80% of the users. If that doesn't work, they'll just have to ask their ISPs to block traffic to facebook and their associated IPs. It's not like they can't just install DPI equipment.

Wel, they could, but ISP's (as well as regulators btw) are fairly reluctant to block 'popular' services. Companies generally use the legal system to fight back. They don't go around just ignoring laws when caught or scoff at verdicts. Facebook has already said they will comply but appeal.

These are some pretty tall assertions, what evidence is there that Belgium lacks the legal tools to enforce this?

That they didn't?

they could go after advertizers doing business in Belgium. They could go after their payment processor. They could involve EU authorities.