I haven't created a new FB account yet, but I've been thinking about doing it lately. I'm pretty sure I'd hold onto my old one for quite awhile. It might be kind of a hard thing to do though, having two FB pages, since I'd have to be on a constant watch to make sure I don't accidentally out myself on my old page (I'm out to most of my friends, but my family doesn't know at all).
But yes, I completely understand what you mean by "that 50/50 mix of depression and dysphoria." I'd much rather be on some other site (Susan's, tumblr, etc.) where I don't have to hide my true self from the world.