I had something like this happen in 2012, and by posting it on HackerNews, I had a bunch of Facebook people replying - and fixing it. I was simply running a fan page, but when someone else (the official page I think) came into existence, it took all my audience and ported them over to the new one without warning.

I think it had like 300,000 likes roughly. I believe I was one of the few to also manage to monetise a Facebook page way back then using a new startup nobody had ever heard of (at the time) - Teespring! (now Spring, and a YC company to boot). Fun times, but teeth-clenching when Facebook decides to do whatever Facebook decides to do.