You may have picked up on the fact that I'm lukewarm on Facebook in terms of its usefulness for building community or marketing. And let's face it, marketing is what drives many writers over there.
But you know what Facebook has been great for? Finding Kindle deals. I happen to be friends with a large number of writers and bloggers. Many writers will let their Facebook friends know when the Kindle editions of their books go on sale. Many bloggers will let everyone know when books they particularly like are part of a Kindle promotional deal.
This is a totally unexpected benefit to being on Facebook for me, particularly since I didn't get my Kindle until this past year. But right now hearing about sales is probably my favorite Facebook perk.