I don't find that to be the case either. Anecdotal, obviously, but I've worked for companies that range in size and I never add anyone from work on Facebook. Same goes for my wife.

We joined Facebook in the early days when you needed a .edu email address to join, so I've got a lot of college partying photos up there. I've always seen it more of a private space. Not sure if the time you join and your original perception of the service changes your use.

I am very methodical about when I add coworkers to my (private) Facebook profile: the day after I leave.

I see my coworkers for approximately 1/3 of my entire week. If I have a question for them it can wait until tomorrow/Monday. I have nothing to gain by them having access to photos of me at a bar with friends or pictures of me at a half marathon or something.

I would be very unhappy if a coworker added me on FB immediately after joining, and I'm the kind of person who's okay with having some coworkers as FB friends. I probably don't know them, and now I'm in a position where I need to reject an offer of friendship from them.

Screw that.

Hell no.

Like the other replies, I do not do this. I work in the .edu sector if that makes any difference.