Because most users view Top News rather than Recent News in their newsfeeds, brand messages are less visible. This is because Top News shows the news feed of those with whom each user interacts with most frequently. If you like a brand but don't continue to interact with the brand's page, the brand messages won't show up in your news feed.
This is good for Facebook users who want to see the most relevant updates in their feeds. But its bad for brands who are trying to engage people in a dialogue. At least it is a limitation until the dialogue is there.
Facebook's answer for brands - sponsored stories. Reactions? How will you feel about seeing paid ad placements in your news feed?