There is also PubPeer, which allows anonymous commenting on articles, including arXiv preprints. It has been fairly successful -- its commenters have uncovered

There is also PubPeer, which allows anonymous commenting on articles, including arXiv preprints. It has been fairly successful -- its commenters have uncovered several cases of fraud and appear to be fairly active: https://pubpeer.com/publications/DF28B786257B8916013EC2F9CC5360 https://pubpeer.com/publications/DF28B786257B8916013EC2F9CC5...