In her July 6, 2012, blog post via LinkedIn, "An Overview Of The NLRB's Most Recent Guidance On Social Media And Confidentiality Policies," (free LinkedIn membership required) Karen Luchka of the Columbia, S.C., office of Fisher & Phillips, LLP, does a great job of translating the accumulating bits of legal guidance that have come out of the NLRB.
The three reports she references can be found on the NLRB website at these links:
Memorandum 11-74: August 2011
Memorandum 12-31: January 2012
Memorandum 12-59: May 2012
The first two memos discussed various charges related to employer-employee disputes over Facebook usage. The newest memo covers seven cases where the Board evaluated employer policies on social media. In six of them, the Board found the policies overbroad, but in the seventh, the policy was upheld.
Acting General Counsel Lafe Solomon even included the validated policy as an attachment to the May 2012 memo.
With Luchka's coverage of the memo, you have a good summary, but I recommend reading the memo for yourself and especially comparing your social media policy against the one--the only one so far to my knowledge--that the Board validated. And remember: NLRB guidance is not just for employers with unionized workers!