It appears as though Facebook experimenting with new formats of news feed 'announcements' to escalate noteworthy, relevant content to your "top news" feed? This is the first time I have seen notifications like this via my Facebook Feed: essentially noting that two friends of mine have commented on an active post. It's interesting for a few reasons:
First, Dave McClure is a Facebook friend of mine as well... so his article already appeared in my feed. Second, the assumption must be that I am more likely to find it interesting and contribute to it because friends have actively participated. In fact, the newsfeed had already shown me McClure's post and then, on my second visit to Facebook, I got the "your friends commented on Dave McClure's link" alert. Good assumption by Facebook by the way.
These updates are usually reserved for the "most recent" feed and are tied to friend requests, calendar RSVPs, or wall-to-wall posts. I have not yet grouped alerts of friends who commented on a thread... interesting.
Also, very much worth reading Dave McClure's actual post - rather than my analysis of how it appears in my Facebook feed!
The original Dave McClure post which appeared in my Newsfeed prior to the getting the above feed post: