This question has more to do about where your target market is. I know for myslef facebook is useless i make more conversions of of directories like gumtree than facebook. This is just because this i where my market is. I have heard rumours that facebook ads are in genral not so good when compared to google adwords. Though i still firmly believe your site content and if you are advertsing in the correct manner with the correct strategy is more important than the platform of advertising itself.