Community Notes
Twitter's/X's crowdsourced fact-checking (formerly "Birdwatch", 2021). Users write context notes on posts; a note becomes publicly visible only when users who usually disagree rate it as helpful (the "bridging" algorithm).
The bridging algorithm is the key: a note needs approval across ideological lines, not just the most votes → hard to hijack along partisan lines. Open source + open data. Elon Musk highlighted it as an alternative to central fact-checking after the Twitter acquisition in 2022. Meta announced in 2025 it would replace third-party fact-checkers with a Community-Notes-like model on Facebook/Instagram. Criticism: notes often arrive too late (after the viral spread), cover a fraction of posts, and can be gamed on sensitive topics.