This is probably worth noting, for anyone who still uses Facebook, and thinks posting “anonymously” in a group, is actually anonymous. This is especially true for small friend or family groups, and could cause major embarrassment.
Facebook introduced the anonymous posting feature in 2017, allowing users to share content within groups while keeping their identities under wraps. This feature is available on all platforms that Facebook is available on and syncs across all your devices. Whether you use an iPhone, Android, or one of the top Chromebooks, you can share your thoughts anonymously in Facebook groups.
You must be a member of a group that activated anonymous posting to use this feature. While your posts remain anonymous within the group, the admins, moderators, and Facebook have access to your name and profile picture. This measure is in place to maintain group safety and uphold Facebook community standards.
So, if you have a friend or family member managing your group, they ARE going to know what you posted. This is pretty typical Facebook though, who blur the lines between privacy, anonymity, and E2EE. Those are actually binary terms – they are either on or off. You don’t get partial privacy, anonymity, or E2EE (the E2EE I’m referring to is the attention being drawn to the WhatsApp E2EE whilst actively passing the metadata upwards to Facebook and 3rd parties).
See https://www.androidpolice.com/facebook-anonymous-post-tutorial/