That's essentially what's happening when employers snoop on Facebook profiles. They're doing so in order to secretly cross the personal-professional boundary and catch a glimpse of the otherwise forbidden realm of the employee's personal life. Employers are exploiting the employee's incompetence with Facebook settings to see what the employee didn't mean for them to see.
I think that's unethical.
Meanwhile, employees need to take steps with social networking activity comparable to what they do in the Real World.
Employees probably wouldn't post pictures in their cubicles or offices showing themselves drunk and shirtless. Friending co-workers and the boss with similar pictures posted on Facebook isn't a good idea, either, and for the same reason.
Another "gotcha" with Facebook and other social networks is the six-degrees-of-separation problem. Even if you don't friend a boss or coworker, the content of your posts can be copied and pasted and passed along. The more inappropriate and scandalous the more likely that is to happen.
The hundreds or thousands of people who have been fired, reprimanded or demoted for Facebook posts almost always say the same thing: They didn't know their messages could be read by people outside the intended group of "friends."
Yes, you could argue that Facebook posts are public, and therefore fair game. But the people posting them don't think of the posts as public.
Google CEO Eric Schmidt recently made headlines for telling the Wall Street Journal that he believes that in the future people will change their name as part of their passage into adulthood in order to "disown youthful hijinks stored on their friends' social media sites."
That's not going to happen. What will happen is that everyone will get wiser and Facebook and the other social networks will evolve user protections so these things happen less frequently. Google Orkut, for example, fixed this whole problem with sweeping changes to its social network.
In the meantime, Facebook should be fired from the workplace altogether.
Employers shouldn't friend and follow their employees for the purpose of crossing the boundary between personal and professional. That boundary should be respected on Facebook just as it is in the Real World.
And employees shouldn't talk about work or co-workers, and should be very careful about whom they friend and what they post.
Facebook is a murky, confusing gray area for many people. And that's why I believe it's unethical for employers to take advantage.