Yes, all advertising will be filtered through Google, Facebook and someone like a Verizon (or whichever Telco can weaponize its data best, currently looking like Verizon).
What that means is that sites like Reddit can still make ad revenue, but only by filtering it through one of those players. That could mean using Google's ad server tech, or integration with Facebook's audience network, or whatever Verizon comes up with.
But yes, most digital ad revenue currently flows through those few players and that will only accelerate.
The true future of Reddit and other publications is as marketing vehicles to sell stuff directly to their readers/audience. Ads are basically just a stopgap along the way to content-driven commerce.
This also seems like something ripe for abuse. You use Google's AdNetwork because it's the only way to access revenue for advertisements. But google can keep upping the cost of it and companies are left with no choice but to pay it or fold up shop.
Maybe this is a good thing and companies shouldn't be relying on ad revenue. This is basically what I tell everyone who comes to me with a startup, mobile app idea. But at the same time, there are companies that provide real value that don't have a lot of other good avenues for revenue.
And in the meantime if it's Facebook, Google, Verizon, Snapchat or nothing, what happens to all the other apps and webpages currently only surviving on meager Ad Revenue if advertisers abandon them wholesale? Can something like Reddit even exist when it makes very little money on it's own and can't subsidize with Ad revenue?