Phil Corbett, The New York Times associate managing editor for standards, tells me in a phone interview why the paper chooses to have only informal social media guidelines:

We have not done a very formal, detailed written policy. We've talked about it, but up until now we've made a conscious decision not to do that. Partly because we've really been encouraging our journalists over the past couple of years to embrace social media, to use it as a tool, to get comfortable with it.

We think it's really important for them to do that, and we've been concerned that if on the one hand you tell all your reporters and editors "Social media is great, you really should be experimenting and getting the benefit of this great tool," but on the other hand, "Here's 27 rules that you better not violate or you're going to be in big trouble" -- that's not necessarily the most effective way to encourage your journalists. ...

We do talk about it a lot. I talk to new people who come on board, and to reporters and editors who are getting more deeply into social media. We have social media editors and producers who are available to work with our journalists to help them and to give them advice and guidance. ...

But in general our message is that people should be thoughtful. They need to realize that social media is basically a public activity, it's not a private activity, and that people will know that they work for the Times, that they are Times journalists, and will identify them with the Times. And so they should just keep that in mind and be careful not to do anything on social media that would undercut their credibility. ...

So far this approach seems to be working for us. People have been smart about it, and thoughtful.

Earlier: John Paton's three "rules" for journalists using social media (Digital First) | NPR’s new guidelines for using social networks: "Respect their cultures" (Poynter).