I'm so sorry for your loss and that you learned of her passing that way.
Also, I completely understand- I found out about the passing of a relative via facebook before my parents had a chance to call me- some people just don't think.
I'm sorry to hear of your loss. Your great grandma does sound like an amazing person.
I was seething two years ago, when my cousins posted things about my grandfather's passing on facebook before we had a chance to call his siblings. I absolutely lost my shit.