For those of you who have successfully self-published, which social media sites have been the most useful parts of your self-marketing platforms? I realize that this depends heavily on your genre, audience, and number of contacts online, of course, but I'm still curious. The idea of trying to establish a meaningful presence on every single social media site I can find (without just posting the same content to all of them at once) seems a bit overwhelming. I've seen an awful lot of advice saying that an Amazon author website, a Wordpress blog, and Facebook and Twitter accounts are important. What about the others--Instagram, Goodreads, Tumblr, etc.? If I want to create a serious presence online and yet still have time to write my actual fiction and do my day job, where are the best places to expend some effort, and which have been more dispensable for you?