"attaching a link associated with at least one of the activities of another user to at least one of the plurality of news items where the link enables a viewing user to participate in the same activity as the another user"
The github news feed includes notifications of comments, and there is a link that can be used to go to the comment page and also comment. As I'm reading clause, the link itself doesn't need to actually cause an action, and the link in that github news item "enables a viewing user to participate in the same activity as the another user."
Also, you can edit and make commits from the web interface, so this could fall under this as well.
The github news feed includes notifications of comments, and there is a link that can be used to go to the comment page and also comment
Ah, that would likely infringe. They could easily get around this by removing the direct link to comment and having users view the comment thread before commenting. Or have a text box instead of a link, where the user would enter their comment directly in the news feed.
And realize that if github and other social networks have similar features that were added after 2006, they are infringers, not prior art.