Oh, I've noticed that, too!
Several years ago, I met a FtM friend at work and noticed he had both a male and female FB account, so I got brave and created a female one.
I didn't realize that all of my friends and family from my male account would see it, too! I think at this point it's become more distant, and I don't think it would out me to *most* people. Only a couple of years ago did I put a real FB picture, but it is so obscured by a rainbow that I'd be shocked if someone knew, even as paranoid as I am.
It's my mom I'm worried about. I'm using the name she told me she would have given me had I been born female. If she were to see it, she would know it's me instantly. I feel like a sitting duck, but I don't want to remove the FB or change my name to something else because it is the real me and it makes me happy. I am barely even active on my male FB anymore. I'll accept friends, I'll respond to messages, but I practically never post. I post all of the time on my real account. It makes me happy.