Errr that’s not really how it works. Meta sells ads and conducts business in Canada. They have Canadian entity which is required to conduct business in Canada, and must comply with local laws to be permitted to conduct business there
Are Canadians not allowed to buy from foreign companies?
The citizen takes on the responsibility of making sure the legal process of doing business in Canada is upheld. EG, they cannot buy something that is illegal to own in Canada, and the relevant taxes and tariffs must be paid, all of those things.
Should Canada be placing tariffs on Facebook? That would be interesting to see.