starting date of collec- tion T0, time span T expressed in days (d) or years (y), number of unique contents C, number of users N and coverage nc (fraction of users with classified leaning). For Twitter, T represents the window to sample active users, of which we retrieve all the tweets related to the topic via the API (more info in SI). Media Data set T0 T C N nc Twitter Gun control 06/2016 14 d 19 M 3963 0.93 Obamacare 06/2016 7 d 39 M 8703 0.90 Abortion 06/2016 7 d 34 M 7401 0.95 Facebook Sci/Cons 01/2010 5 y 75 172 183 378 1.00 Vaccines 01/2010 7 y 94 776 221 758 1.00 News 01/2010 6 y 15 540 38 663 1.00 Reddit Politics 01/2017 1 y 353 864 240 455 0.15 The Donald 01/2017 1 y 1.234 M 138 617 0.16 News 01/2017 1 y 723 235 179 549 0.20 Gab Gab 11/2017 1 y 13 M 165 162 0.13 what-is-pushshift-io/) at this link https://files.pushshift.io/gab/. Red- 401 11. AL Schmidt, et a 3035–3039 (2017 12. M Cinelli, et al., T 13. E Bakshy, S Mes facebook. Scienc 14. KH Jamieson, JN establishment. (O 15. RK Garrett, Echo news users. J. C 16. K Garimella, G D media: Echo cha 2018 World Wide Steering Commit 17. K Garimella, G D Attention on Con Web Science Con 18. W Cota, SC Ferr information sprea 19. JT Klapper, The e 20. RS Nickerson, C psychology 2, 17 21. M Del Vicario, et book. Sci. reports 22. A Bessi, et al., Sc one 10, e011809 23. CR Sunstein, The