LJ has thriving communities. It has creative writing and art and actual discourse on topics. Facebook has brief quips and lots of advertising. LJ is helping to keep the art of conversation alive, and Facebook is basically on online forum for text messages.
And when you complain about it, everyone says "yeah, but this is easier." *grinds teeth*
ETA: I do get that Facebook has some uses, such as locating lost friends. What galls me is that so many people who used to use LJ seem to have "left for FB." It is not a reasonable substitute.