Commenotes: Synthesizing Organic Comments to Support Community-Based Fact-Checking

Community-based fact-checking is promising to reduce the spread of misleading posts at scale. However, its effectiveness can be undermined by the delays in fact-check delivery. Notably, user-initiated organic comments often contain debunking information and have the potential to help mitigate this limitation. Here, we investigate the feasibility of synthesizing comments to generate timely high-quality fact-checks. To this end, we analyze over 2.2 million replies on X and introduce Commenotes, a two-phase framework that filters and synthesizes comments to facilitate fact-check delivery. Our framework reveals that fact-checking comments appear early and sufficiently: 99.3\% of misleading posts receive debunking comments within the initial two hours since post publication, with synthesized \textit{commenotes} successfully earning user trust for 85.8\% of those posts. Additionally, a user study (N=144) found that the synthesized commenotes were often preferred, with the best-performing model achieving a 70.1\% win rate over human notes and being rated as significantly more helpful.