I'm bisexual and usually I keep it to myself. Most of my friends didn't know until thy caught me on the phone to my ex-girlfriend, asking who it was etc. Everyone assumes I'm straight and that just I'm incredibly androgynous from my behaviour, my attiude, outlooks and the way I dress etc.
My Mum asked me if I liked girls and I was honest with her, nevertheless it was a huge mistake and it's not been brought up since, I basically got the 'it's a phase' speech. Anyway, back to my point
I don't usually tell someone unless they ask, and I have lied about my sexuality and said I'm straight to some people because I'm scared of certain things, but other than that I keep it private.
I don't even say 'I'm bisexual' to people on Tumblr, I don't have to. I can post what I like and it's seen as okay. Mibba I'm completely open about it though, it's hard for me to keep it secret, especially since it's the only place I can truly discuss it with another person.