Remember the first rule of Facebook, they will do anything and everything to get you to stay and interact on the site. That’s what this new feature is all about:
If all your friends are reading it, you wouldn’t want to miss out, would you? You would want to read it too, so you can stay up to date with the relevant conversation, right?
That’s the kind of push Facebook’s going for – there’s no further information available, and you can’t click on the note to see which friends, specifically, have read it. But it lets you know that some people you know have, which could prompt more users to check out the same and ensure they stay up to date within their circles.
It’s the same general idea behind showing you when your friend “likes” or comments on a post, surely you want to check that out, right? Only now, they’ve taken it a step further, now they’ll tell you some of your friends have clicked through and read articles that were published from elsewhere too. It’s probably a good thing that they aren’t telling you which friends, that could get embarrassing.