I'll try and keep this as brief as possible without drip feeding.
Been with OH for 3 years and have DD aged 1. Both have other children. I maintain a friendly enough relationship with the ex. Talk about only the children, no connection on social media etc. OH has one DS8 with ex who also has 2 older children (not related). My ex is pleasant enough t my OH, hello or smile etc but nothing else. His Ex is rude and has never spoken a word to me, despite asking us to look after one of her other children overnight when she couldn't get a sitter (literally happy to leave him and we had only been together about 8 months and had never actually seen each other never mind met). And despite me being in her DS life at least 50% of the time. oH says he can't stand his ex and they only have contact over DS. He has never been on social media with her and any pictures of DS are shared on WhatsApp.
So that's the backstory. The other day I was messaged from DSis in law. She was laughing at something his ex had posted so I had a look, when I clicked on it said 2 mutual friends and one was OH. Turns out he had friended her over a year ogo. I know it seems silly but it feels like such a betrayal. She is a very prolific poster of bedroom selfies etc. So she has been able to see every interaction we have had. Every picture of my daughter etc. we got engaged back in may and he asked to hold off from putting the news on social media until he had told people face to face. We had told al his family but he still wanted to hold off, I now think so he could tell his ex. He also decided he wanted a tattoo and messaged me with some ideas on 6th April. She posted on 6th April her new tattoo. So he is definitely interacting (although he claims this is a coincidence and he was already thinking about getting one and had just not
Mentioned it). He says they are only friends on Facebook to share photos of their son (which there aren't many as it's all bedroom selfies) and previously this has been done over WhatsApp so I don't understand why he would need to be on her Facebook. And if he did why he has hidden it from me both by not mentioning it and hiding when he has been tagged (granted there are only 2 or 3 tags). He says that it isn't lying when he just hasn't mentioned it, I say lying my omission is still lying. So I am prepared to be flamed for feeling so petty over social media, but really I feel so betrayed that our lives and relationship have been playing out in front of her and I didn't know.