Disqus commenting platform is adding support mentions within comments. "If you've ever mentioned someone on Twitter you'll be right at home with Disqus's implementation."
"Start typing a comment, then type the "@"symbol whenever you'd like to tag someone -- this could be a user who has left another message in the comment thread, or a user who hasn't participated. The convention is very similar to the "@reply" system popularized by Twitter, and a small overlay will pop up with autocompleted names drawn from both Twitter and Disqus. Tagged names appear with a gray box around them in the published comment, which looks nice."
Disqus will also send the tagged user an email notification or a Twitter @reply, informing them that they've just been mentioned in the comment thread. This gives you an easy way to invite someone to comment on a thread that you think they'll be interested in or knowledgable about.
Facebook, which now competes directly with Disqus via its Comments social widget, will probably add its own version of tagging in the future.
Another compete Livefyre, has also offered mentions using your social graph for around six months now.