Just read a piece by Scott Rosenberg warning that journalists shouldn't rush into Facebook. He's right, but imho his analogies don't quite capture the danger.
Now, what if Facebook made a deal with the Chinese government? Would you feel comfortable publishing a story unfavorable to China on Facebook? Would you be surprised if Facebook altered it, or took it down, or got you fired? What if China owned 10 percent of Facebook?
Would you be surprised if, in a few years, Facebook wasn't drawing from the talent pool of the US government, or the largest banks and corporations and other Silicon Valley firms? Of course they already are.
If Facebook lasts, and if it grows like most companies do, they will hire from the general talent pool from all sectors of government, academia and buisness. The kind of freedom you'll have on Facebook won't be anything special. It'll be a corporate-owned or controlled place like a shopping mall, or the Smithsonian. Subject to all the political pressures of large entities. I don't think that's a good environment for journalism, reading or writing.
Update: Ars Technica provides all the evidence you need.