There have been a number of legal developments involving Facebook, MySpace, and Twitter lately, all demonstrating that the intersection of traditional employment law and social networking sites has yet to be fully mapped.
Part of the problem seems to be that users of these sites believe themselves to be invisible, at least to their employers. For example, according to an article on Workforce Management (you may have to register to view it), investigators looking into employee workers' compensation claims search social networking sites for photos of employees engaged in activities that are incompatible with their claimed injuries -- such as bowling a perfect game, taking judo classes, or riding a bucking bronco. Then, there's the recently reported case filed by a Canadian woman, who says that her sick leave insurance benefits for depression were improperly cut off after an agent for the insurance company found photos of her on Facebook vacationing and taking in a show at Chippendales.
Even employees who take precautions to make sure employers can't view their posts are finding that management has its ways. In a recent case in the District Court of New Jersey, for example, some employees at Houston's restaurant created a group on MySpace for the stated purpose of venting about their jobs. The group was private and could be joined only by invitation. However, an employee member of the group showed it to a manager (she testified that she felt pressured to do so), a number of managers read it, and the employees who set it up were fired. The court recently upheld the jury's verdict in favor of the employees.
Some companies are so concerned about what employees -- or even the friends of employees -- might be saying about them online that they have instituted content rules or outright bans on social networking. According to an article in the National Law Journal, more than half of the companies responding a survey said that they prohibit employees from visiting social networking sites while on the clock. And, some companies have adopted rules about the content of employee posts. The Associated Press, for example, is reported to have not only set strict rules for employee pages (including that they should not express political affiliations or take a stand on contentious issues, even if their pages are restricted only to friends), but also asked employees to police the content others post on their pages. (You can find an article from Wired about it -- including a link to the actual policy -- here.)
To learn more about why your company needs a policy on employee use of blogs and social networking sites, read Nolo's article Employee Posts on Facebook, MySpace, Twitter, and Blogs.