According to facebook:
We determine whether something is a top story based on lots of factors, including your relationship to the person who posted the story, how many comments and likes it got, what type of story it is, etc. For example, a friend’s status update that might not normally be a top story may become a top story after many other friends comment on it.
So, what was once top news, is now a top story and if you don’t want it as a top story…
WHAT DO YOU DO?
Unmark it as a top story.
If you see something that isn’t interesting in top stories, hover over the right side of the update in question, click the arrow and click ‘Unmark as top story’ from the menu.
Stories like the one you un-click will not appear next time. Eventually, there won’t be many perimeters for them to tag “top news” and they may disappear all together.
Personally, I like the changes top stories.