I figure if someone recognized you on the board, that means they're on the board too...
as soon as I joined this board, my BFF from high school recognized me. She sent me a PM on FB. I think it's fine - we're both TTC, neither of us are really telling anyone, so it was actually kind of nice to have someone to talk to IRL. We actually talk a lot over FB, and don't interact here very often.
My SN is fairly recognizable, I've used it for a few online message boards, but I'm also pretty much an open book, so if someone finds out I'm TTC, I'm fine with it.