Short answer
Not in a steady slide. Engagement per follower, the closest public signal of reach, fell by about 25 to 35% from 2016 to 2018, then recovered to roughly its 2016 level by 2020.
What the evidence shows
- The study uses 506,179 posts from 112 party pages and 74 party leaders' pages in 20 European countries, January 2016 to September 2020, comparing each page with itself.
- Interactions per post per follower reached a low in mid-2018 and then rose by about half. The fall and recovery were found on most pages, on party and leader pages, and within each type of post.
- There was no sudden fall at the News Feed changes of June 2016 or January 2018: the decline was gradual.
- It was not a mechanical effect of growing audiences: within pages, interactions rose roughly in proportion to followers, which grew by about 93% while total monthly interactions grew by about 89%.
Caveats
Engagement is not reach. The best-known fall in organic reach, 2012 to 2014, lies before page-like counts were recorded and cannot be tested here.