Facebook makes a very poor job of keeping its users informed of changes they make to your account. This case in point. The default setting on your news feed is now only seeing feed from those friends who you interact with on a regular basis.
So if you wondered why someone you know had seemed to 'disappear' faster than Hosni Mubarak it is because you have not intereacted with them enough. Now some may argue that the point of social networking is to interact and therefore it makes sense for Facebook to have done this. If you are in this camp, then you do not need to do anything.
If you do want to see all your friends news again go to this blog post by Dan York at Disruptive Conversations to make the changes necessary.