Image via WikipediaGmail came up with the idea of a Priority Inbox. People have loved it. Finally a cure for the common cold, aka too much email.
The stream is Facebook's version of the inbox. And it became cluttered a long time ago. Facebook does use algorithms to determine what to put into your news stream. But I think that sill leaves room for a Priority Stream. Algorithms are too Google Search, they are not Facebook.
You should be able to pick the 10 people whose updates you don't want to miss at all. I guess Gmail already had filters. What you can do with the Priority Inbox you could have done with filters before, so they claim. But it is not the same thing. It is like saying Geocities already could have done much of what Blogger later did, but it is not the same thing.
Facebook already makes it possible to follow some people more closely. But it is not the same thing. You want a Priority Stream next to your regular stream that you see as soon as you log in.
Twitter could argue it already has lists. But there also a priority stream would make sense.