I'm being all calm and explanatory in real life, but honestly my DM just doesn't get the peripheral nature of Facebook. She seems to feel that someone posting things that she isn't interested in is the equivalent of someone constantly engaging her in conversation about a subject that she's not interested in and not shutting up. So she complains about other people's behaviour, like my cousin posts too many photos apparently. 
I deliberately didn't add her as a FB friend. She sent me a friend request, which I accepted to keep the peace. The she complained that she was seeing too many posts that were nothing to do with her. I'm pretty active on various special interest FB groups so she was seeing things that I had put up about quilting or crochet or Strictly. So I restricted what she could see.
Then today one managed to slip through my restrictions and she phoned me in a huff to ask why I hadn't told her that I'd had a three-day stomach bug. I hadn't and told her so, so she asked why I was lying on FB. Eventually, after a lot of searching, it turned out that one of my FB friends had posted about not being sure a quilt would be finished in time because he'd had a stomach bug and I had commented underneath. So I explained that, but apparently it was definitely me that said it and not someone else and she's going to take a photo of it to prove it to me.
So I've double checked the exclusions and made them stronger. I'm thinking about unfriending her and claiming it was a FB glitch if she notices.
No advice needed (and I'm pretty sure that IANBU), I just feel better for having off-loaded that.