There's Facebook and Twitter, Pinterest and YouTube, Digg and Tumblr, LinkedIn and Blogger . . .
Head spinning yet?
Don't worry, it's normal if you've looked at the number of social networking sites and wondered, Which one should I use?
It doesn't matter, as long as the social media you're using is furthering your message and you're using it with excellence. Don't worry about trying to do them all. If you tried, you'd probably spend most of your time trying to learn and use them all and not accomplishing any writing at all.
Okay, so which one should you use?
I'm a little partial to Twitter, Facebook, YouTube, and Blogger, but that doesn't mean you have to be. Use whichever you're comfortable with.
Then, use them consistently and with excellence.
Don't have any?
Start with one. Then, when you're comfortable with it, add another.
I read an interview once with a literary agent, who said that when she's researching a new client, she actually searches out to see if they're using Twitter and Facebook. She also said that every author should be using Goodreads too, so that might be another one to add to your arsenal.
But if you're going to do it, learn to do it well and with excellence.