I an firm believer in the power of social media as a marketing tool, and I think law firms should not take the drastic step of banning their employees from using Facebook. I also think lawyers need to be especially careful about their privacy settings to make sure their pages and photos are not publicly accessible. Even if you are using Facebook at work, be careful about which coworkers you add to your friend list.
Unless you have a great relationship with your boss, do not add them as a friend. Just because you get along with your boss, or even occasionally go to happy hour with them, that does not mean they need to see every aspect of your life (and vice-versa). All it takes is one awkward drinking picture on your page, or on theirs, and things can get weird. Besides, do you really want your boss commenting on your status over the weekend?
If you just have to be facebook friends with your boss, there are a couple things you can do to minimize the risk of something going horribly wrong. One, change your privacy settings for that particular person, so that they can only see your basic info–no wall updates, no pics, etc. Two, you can maintain a relatively civil facebook page, and not have crazy pictures or wall postings on your page. For many people, however, that is the fun part of facebook. If you are that person, then either do not friend your boss, or only let them see certain parts of your page.
The same concerns are true for friending other coworkers, although probably to a lesser extent. Most coworkers do not have the power to fire you. Then again, there are plenty of tales of coworkers tattling on someone else to the higher ups. If you have any hesitation about adding someone you work with, just play it safe, and say no.