Well, our mothers were there. My brother was there and his brother already knew. On the way home, I called my closest cousins and closest friends and then I immediately posted it on FB. However, my mom is one of 14 and my family is HUGE! There is no way that I could call everyone so FB was my way of telling them.
My FI didn't tell his closest cousin and she found out on FB. LOL! She called his mom and was like umm....I read on FB that....My FMIL said, "She put that on FB" His cousin's reply was, "Yeah, that's what it's for." LOL! I love it.
I do, however, think that you may have considered waiting to post until you told those who you wanted to tell directly. Just apologize to your grandmother. Let her know that it meant a lot to you to tell her face to face and that's why she didn't know yet. I hope everything works out. Keep us posted!