(The possible exception to this is paid promo on Facebook, but I consider that a category of advertising, rather than social media – since the whole idea of social media is being social, ie: interacting.)
I do maintain something of a presence on Twitter and Goodreads, but I try to share stuff or talk books/track my reading process on those two sites. However, on Twitter I do also love to share the art for my book covers and will over indulge in that 😀 I also post a few links to this blog, but mostly check out what my fellow writers are up to.
There are various book promotion services geared toward twitter and such – two I’ve trialed over the years are AskDavid and BooksGoSocial and will report on them soon. It can be quite difficult to effectively track such promotional material for sales results.
If you’re unsure of whether or where to start with social media, I’d recommend only using a social media platform or two and focus on them rather than spending time trying to cover fifteen different bases. Stick to a few you enjoy, be approachable and don’t get sucked into the horse-radish that passes for ‘debate’ on such sites and you’ll probably find social media fun, though perhaps not essential for marketing.