The Public that Engages Invisibly: What Visible Engagement Fails to Capture in Online Political Communication

Measurements of political polarization online have so far been largely focused on visible traces accessible through platformAPIs,while neglecting invisible traces not recorded or otherwise unavailable via online channels, which can reveal key aspects of political engagement online. Our study aims to address this gap by investigating the polarization measurement bias that arises when only visible engagement is considered. Using a combined dataset that links survey responses with YouTube digital traces froma representative sample of Hungarian Internet users (N=758), we uncover disparities at both user and channel levels.We find that users who visibly engage through commenting are more politically polarized. People tend to comment and subscribe to ideologically concentrated content, while viewingideologically diverse content. We also notice that ideologically heterogeneous channels are more likely to share viewers than subscribers or commenters. Thus, the segregation ofpolitical channels may be overstated or simplified when relying solely on public comment data. Our results suggest that research using only visible engagement may overestimate the extent of polarization and the prevalence of echo chambers online. We highlight the benefits of data donation to address measurement bias in online political communication, and contribute to the polarization literature by providing a fresh evaluation of potential biases in platform-focused research.