Via Alex Roddie, I came across an article on Cult of Me that offers some thoughts on online advertising. This is one of many things that independent writers need to consider, and there isn’t much data about the effectiveness of different sites and advertising methods.
My own advertising has been limited to Goodreads and Facebook. Goodreads is expensive, as Michael Brookes says, and probably not all that effective. Facebook has a nice interface and allows you target your advert so precisely that you’ll feel dirty. (I did, and left Facebook shortly afterwards.)