I would say do what you are comfortable with. If you are comfortable making a formal announcement on Facebook, then do it. It may make things easier down the line, but then it may complicate things too.
Just know that not everyone will be okay with the news. Some people will not understand or not agree with the idea of trans. People all have their own opinions.
But what's most important is that you feel comfortable in who you are and your body. If someone doesn't agree or is nowhere near supportive, they may not be someone you want to be around for awhile.