Of course Canada has the legal authority over Facebook's activities in Canada. Facebook not having any datacenters there just makes enforcement slightly harder.
What activities do they do in Canada?
Provide advertising services, which I'm sure they happily exchange for $CAD
Arguably that's Canadian companies doing business in America, not the other way around.
Either way, Canada has jurisdiction.
They do sell to Canadian companies, hence they are presented there.
The reason Facebook doesn't have any Canadian data centers is that the Canadian government won't let them opt out of Canada's privacy laws.