And now %X of that cost will return to the economy instead of going overseas. Ad costs should stay roughly the same, since they are the result of bidding. It's not like Facebook leaves $2 on the table and says we'll keep that there for you guys unless we get taxed.

When something has near zero marginal cost, it is all just about demand. The only scarcity is caused from other bidders.

Why should a country send %X margin on all of their consumer products overseas (via ad costs) to someone who is selling their own citizens' content and capturing the users/locking them in with network effects? There is a lot of innovation at Facebook et. al., but they get a massively outsized portion of the network's value when you compare what they put in vs. what users are putting in.

Where it could hurt would be a service that can barely break even using ad revenue and goes under as a result. But for big corps with network-effect lock-in in their segment, it seems like a net win for any country that does this.