Which social network(s) should we be using?

With so many different social media platforms, it's hard to decide which one(s) you should actually join; should you have a profile on all, or just select a couple to focus on? Being an artist – trying to build an audience – this decision is even harder. Worse, when you decide to create a new profile you're daunted with questions like: What should I post on this site? What will make it relevant?

For this reason, I've decided to use each site a little differently.