Don't be so sure your experience matches everyone else's. The 'we' you mentioned does not include me.
I will add all my work contacts on LinkedIn but I will only add a few (about 5%) on FB; ones who I socialise with outside of work.
It seems very customary to add at least everyone on your direct team to Fb though.
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.
Wise choice, although I'm sure she has other ways of seeing what you get up to.
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.