I agree, personalized propaganda is a nightmare.
Some things can be readily compared - easy to compare the Washington Post of today to Washington Post of the 1970s.
But many of today's subreddits, facebook pages and 4chan boards don't have a direct equivalent in the 1970s.
Comparing 1970s WaPo vs 2018 WaPo will give you a different result to comparing 1970s WaPo vs (2018 WaPo + r/LateStageCapitalism + r/The_Donald)
I'd like to subscribe to your newsletter ()!
But seriously, The Birth of a Nation came out somewhat before r/RarePepes. Michael Moore films also (barely) predate social media. And people were reading about JFK assassination conspiracies and fake moon landings way before "new" media was around.
() The equivalent to a niche social media group was subscribing to a niche political newsletter.