I just noticed today that Facebook has started replacing the preview text in user-posted links with ads. Earlier today I posted this
Now, check out the actual page I linked to - http://www.joystiq.com/2012/10/09/zyngas-business-valued-at-nothing-cityville-2-coming-soon/ - notice that the words in the preview text never appear in the article. In fact, the article is pretty critical of Zynga, and changing out the preview text really changes the meaning of the post to someone glancing at it in their Facebook feed.
Of course, this link was just to an article about a sucky game company, so who really ends up caring that much. But, what if it had been a link to some kind of political article, or a news article about an oil spill. Imagine that the opposing party or the company responsible for the spill purchased the keywords needed on facebook and replaced the preview text with their own spin on the topic? All of the sudden, you can start really changing what people see, since I am guessing a lot more people see the previews than click through to the article. Not only that, but the view in the little text preview will be taken by the casual viewer as the view of the poster of the link, so your friend's perception of you can become skewed by advertisers.
I assume that pretty soon Facebook will introduce a "new" service that will make it go back to working the old way...for $7.00 a post.