"Nearly 18 hours after a terror attack that killed 49 people at a mosque in New Zealand on Friday, footage from the shooting remained live on YouTube and Facebook. 'The video is still circulating online,' said David Ibsen, the executive director of the Counter-Extremism Project, an organization that maintains a hashing database for terrorist videos. 'The technology to prevent this happening is available. Social media firms have made a decision not to invest in adopting it.' According to Hany Farid, a professor of computer science at Dartmouth College who has used hashing to combat child pornography, if Facebook were using 'robust' hashing — a method that should be able to detect variations on reuploads — it 'should be finding the majority of reposts.'  Additionally, any variations that fall through the cracks can then be hashed, and added to the same database to prevent further reuploads. 'Hashing has the advantage that it works at scale,' said Farid."