Yes and no.
To some extent, social media is self-fulfilling. When you already have fans, they want to hear from you, and social media is the bridge that they use to connect you to other potential fans. It takes a long time and a lot of giving away free content to build fans, though.
Two ways artists can use social media more effectively:
1) Know the difference between a friend and a fan. A friend supports you and wants you to do well. A fan buys your stuff because they like your work. Liking YOU is different than liking your WORK. Amanda Palmer has created a lot of controversy. Not everyone likes her. But the people who like her work buy it, often on the PWYC model. She's made it work well enough to make a living at it, but there were a lot of long hard hours on the road, sleeping on couches, before she made it. She made contact with people in person, and cultivated them as her fans, not her friends. Her fans still let her couch-crash and host concerts in their living rooms. Her friends get her phone calls.
Some social media is better suited to building friends--Facebook, Livejournal--because they presuppose an intimacy with the personal lives of the people interacting. Other social media is better for fans--Twitter, blogs and Tumblr--because the knowledge base needed to appreciate someone's post is much lower. There's more instant gratification, did-you-see-this-link-it-was-cool factor.
To connect with fans, you must generate content. If the content is good, consistent, and entertaining, your fans will share it for you, and it will build more fans. Friends say "Oh, that's awesome!" and click "like". Fans share it with their networks.
2) Use social media as a reinforcement rather than the first troops. That is, if you are connected, however loosely, with 500 people in the real world (members of your church, audiences at your events, whatever), your social media reinforces that connection. It won't make a stranger buy your book--but it will remind your acquaintance that they were planning to. Or, your friend will recommend your book to their acquaintance rather than buying it themselves.
So most of us are going at it backward - we're trying to create online connections from scratch and hope they transform into real world connections. Instead, we should be meeting as many people as possible in the real world, and offering them something of value (whether that's friendship, fellowship, or good business advice) so that they look forward to hearing from us online. Nobody wants to buy @jane.x.smith435's book they've never heard of. But when they liked Jane that time they met her at a professional gathering, and she's sent them a couple of articles they're interested in (not by her), and they clicked over twice to her blog and laughed about something she wrote, they might mention to their other friend, "Hey, this woman I know vaguely just wrote a book that I think you'd like".
What's your best social media success?