I suppose because they're used differently? I'm not leaving LJ and as you say it's a far superior platform; I loathe Facebook but I'm not leaving that either since it's the only practical way to keep in touch with two continents' worth of friends and relations.

I think it might be because Facebook was never going to be anything but evil, whereas so many LJers still remember when it was Brad and the servers in his basement, and still feel betrayed that it isn't that anymore? I don't know.