DH told MIL not to add me on FB - this was before we even had children - as I wasn't friends with any of my own family on there, he said 'please don't add marshmallow on there, her family don't do FB, it's just her friends and she'd like to keep it that way'.
She still added me anyway, and I felt duty bound to accept. 
Most of the pics of DD I put up there I only share with 'close friends', and MIL isn't in my 'close friends'. I feel a bit bad about it but if I didn't do that she'd like and comment on every single photo and share them with all her friends, most of whom I don't know from Adam. It has made me massively reduce the amount I put on FB now, though - I don't use it for sharing innermost thoughts now, just for stalking people.
I do find the same with DH's best friend and his wife, though, who I don't know very well (they live abroad) - I put them in my 'close friends' group because, well, they are his closest friends...but they 'like' every single photo I put up, and also any photo I am tagged in...it feels like they're watching my every move and it's a bit creepy! I think they think they're just being super-friendly and nice, though.