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To assess the different dynamics, we perform a comparative analysis on more than 100 million pieces of content concerning controversial topics (e.g., gun control, vaccination, abortion) from Gab, Facebook, Reddit, and Twitter. The analysis focuses on two main dimensions: 1) homophily in the interaction networks and 2) bias in the information diffusion toward like-minded peers. Our results show that the aggregation in homophilic clusters of users dominates online dynamics. However, a direct comparison of news consumption on Facebook and Reddit shows higher segregation on Facebook.<\/jats:p>","DOI":"10.1073\/pnas.2023301118","type":"journal-article","created":{"date-parts":[[2021,2,23]],"date-time":"2021-02-23T13:41:19Z","timestamp":1614087679000},"update-policy":"https:\/\/doi.org\/10.1073\/pnas.cm10313","source":"Crossref","is-referenced-by-count":1595,"title":["The echo chamber effect on social media"],"prefix":"10.1073","volume":"118","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-3899-4592","authenticated-orcid":false,"given":"Matteo","family":"Cinelli","sequence":"first","affiliation":[{"name":"Department of Environmental Sciences, Informatics and Statistics, Ca\u2019Foscari Univerity of Venice, 30172 Venice, Italy;"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2415-494X","authenticated-orcid":false,"given":"Gianmarco","family":"De Francisci Morales","sequence":"additional","affiliation":[{"name":"Institute for Scientific Interchange (ISI) Foundation, 10126 Torino, Italy;"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6859-0391","authenticated-orcid":false,"given":"Alessandro","family":"Galeazzi","sequence":"additional","affiliation":[{"name":"Department of Information Engineering, University of Brescia, 25123 Brescia, Italy;"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-4374-9324","authenticated-orcid":false,"given":"Walter","family":"Quattrociocchi","sequence":"additional","affiliation":[{"name":"Department of Computer Science, Sapienza University of Rome, 00185 Rome, Italy"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9161-5339","authenticated-orcid":false,"given":"Michele","family":"Starnini","sequence":"additional","affiliation":[{"name":"Institute for Scientific Interchange (ISI) Foundation, 10126 Torino, Italy;"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"341","published-online":{"date-parts":[[2021,2,23]]},"reference":[{"key":"e_1_3_3_1_2","doi-asserted-by":"publisher","DOI":"10.1073\/pnas.1517441113"},{"key":"e_1_3_3_2_2","doi-asserted-by":"publisher","DOI":"10.1080\/1369118X.2018.1428656"},{"key":"e_1_3_3_3_2","doi-asserted-by":"publisher","DOI":"10.1080\/15205436.2015.1045149"},{"key":"e_1_3_3_4_2","unstructured":"N. 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