Short answer
Mostly nationally, not Europe-wide. About 26% of month-to-month swings in a page's engagement were shared with its own country, against about 11% for Europe as a whole, and party families did not move together across borders.
What the evidence shows
- The study uses 14,510 page-months from 173 party and leader pages in 20 European countries, January 2013 to September 2020, and splits each page's deviations from its own trend into Europe-wide, national, party-family and page-specific parts.
- The country-month accounted for about 26% of the variance, about 15 points above a random-label baseline; Europe-wide months about 11% (3% to 17% across specifications); the party family 3%, no more than random labels give; and the page itself 60%.
- Radical right surges were not synchronised across borders.
- National election windows went with interactions nearly three times as high as usual. The clearest Europe-wide months were the European Parliament elections of May 2014 and May 2019.
- Apparent surges in May and June 2015 coincide with a break in CrowdTangle's capture of photo posts and disappear when it is allowed for.
Caveats
The Europe-wide share is the least certain number, and the pages are the author's own selection, with many more in some countries than others.