To Tweet or not to Tweet

I'm really surprised by The Washington Post's new policy discouraging their reporters from the use of social networks, such as facebook and twitter. In an age where reporters demand transparency and real-time information from other institutions, it only seems fair that newspapers should open up too. It's a great way to get feedback tips from their readers, but the Post's Executive Editor Marcus Brauchli, fears it could harm a reporter's impartiality. “When using these networks, nothing we do must call into question the impartiality of our news judgment. We never abandon the guidelines that govern the separation of news from opinion, the importance of fact and objectivity, the appropriate use of language and tone, and other hallmarks of our brand of journalism," he said. (It's also interesting to note that the full policy isn't available on the web for readers to see.) If you're a working reporter, do you use twitter or facebook as part of your reporting? Is it a threat to a paper's credibility?