Hypothes.is is great for small groups of scientists, academics and they are doing good work . For more general news annotation, I use Fiskkit.com

The only competitor I have seen is Fiskkit (www.fiskkit.com) which lets readers tag and comment at the sentence level and call out logical fallacies

I nominate Fiskkit.com A new site which is a BS filter for news/online content and lets there be a discussion of an article or text that favors facts and logic. Kinda cool. Anyone can tag news article for true/false/bias and nine other things...democratizing civic engagement and disobedience by putting BS filter into hands of anyone. I know they are looking for help to try and root out the crap that passes for news and discourse online and get people more engaged.