I'm a SAHM right now, but before that I worked for a global ocean export line. I am FB friends with most of the people I worked with in our Chicago office, but drew the line at friending managers. I didn't think it was appropriate for them to hear my political opinions, religious views, etc. My supervisor became my FB friend only after I left the company.
I found that when colleagues around the US knew me on a more personal level, we had a better working relationship (although they were pretty good to begin with). Now that I am a SAHM, I keep in contact with most of them because 1) I value them as people and 2) I may need these contacts in the future when I re-enter the workforce.
One of my customers and I have a cool FB friendship. We used to always talk about deep things on the phone, while I was inputting all the details of his orders, so now it is really cool to be able to post and discuss various articles, to see pictures of each other's families, and he keeps me in the loop about how my old company is doing with their customer service.
There was a time when my company blocked FB, because they were certain that productivity was suffering from it, but later they found out that we were MORE productive because we were using the FB chat feature to get things done, since it was more reliable than our intraweb chat.