This is a good thread.
When my book was launched, I tweeted about it all the time. I used to do one liners to 'hook' the readers with the link to my book. I believe I sold precisely zero books using this method, but facebook seemed to be more successful (but that's probably because its my friends, even if I haven't seen them for ages). Now, I have bored myself with it and use twitter to post thoughts or links on topics that interest me (usually education as I'm a teacher by day). No idea if it's led to more sales, but I have a lot more followers than when I started. (by the way, a 'lot' in this post means under a hundred but more than 50!! so take it with a pinch of salt). Oh, and buy my book, please