Multi-Scale User Migration on Reddit.

Migration is a natural phenomenon in online social networks where users move across virtual spaces within or across platforms. Platforms such as Reddit drive migration behaviour by facilitating the formation of communities around discussion topics. However, migration also occurs in relation to the emergence of cyber-social threats. For example, 'trolling' involves users intentionally moving in and out of spaces to provoke conflict. In extreme cases, platforms move to shut down spaces due to user behaviour, forcing those users to migrate elsewhere. This paper provides a set of methods to help study migration at two scales: micro-scale involving individual users moving between spaces over relatively short time periods (between posts), and macro-scale involving groups of users moving over relatively longer time periods (changing their posting habits). We take Reddit as an example platform due to its community orientation, and COVID-19 as a means of qualitatively illustrating the results of our methods. We show how micro-scale analyses reveal sub-network structures indicative of overlapping user communities and how macroscale analyses provide important context to understand the micro-scale patterns, while also revealing consequences of platform moderation on user migration. For COVID-19, the micro-scale analyses reveal an interplay of controversies, politics and humour, while the macro-scale analyses point to increasing politicisation of the pandemic and a rise of conspiracy theories.