So this hasn't officially happened to me yet, but my mom's best friend from high school joined when he became single again and he tells her what my status updates are. BTW my mom doesn't really know what "status update" means. She doesn't even understand what Facebook is.
I mean, she vaguely understands that it's a social site on the internet but she doesn't understand why anyone would join. She also can't understand why people would want to know such boring details about other people's lives like what mundane household task they're doing or what TV show they're watching. I agree.
Parents shouldn't be on Facebook, but some people's parents are and someone made a blog listing all the embarrassing things parents do on Facebook: http://myparentsjoinedfacebook.tumblr.com.