What's the strategy for making advertising on Google and Facebook (+Instagram) - where the majority of all online advertising outside of China occurs - significantly less effective?

Google controls search, Android, Chrome and the Android app store (along with of course YouTube, Google Maps and Gmail, three other giant services). Facebook has its own massive walled off kingdom that exists nearly entirely in apps where they can heavily control ad blocking.

Google nearly doubled its sales in three fiscal years, from $74b to $136b. Something that big doubling in that span of time, is extraordinary. They'll probably hit near $160b in sales for 2019. I'll take the bet that a giant that large, entirely dependent on advertising, will continue to figure out how to insulate itself from ad blocking by controlling the territory around it.

Your premise will always be a non-starter so long as the ad giants control their own sprawling acreage (which they will fight to the death to retain, because it is life or death to them). The ad giants have made sure to control their own destiny, they saw the ad block threat plainly a decade ago or more and moved to stay out in front of it.