I'm telling you to spiff up your LinkedIn profile this week, and I think it's important to have an up-to-date profile when you're job searching. I also think it's important that your Facebook profile (or at least the public face of it) is employer-friendly, and I think there's value in using Twitter to engage in scheduled group chats for professional organizations relevant to your industry.
But I want to be clear that I don't think you're going to land a job solely by using social media. For as many fun stories as there are out there about people strategically using Twitter to get some company's attention and being offered a job, that is not the norm. And you definitely shouldn't simply expect to be "discovered" for going about your daily business on the Internet.
Let's use this example. If your goal is to get cast in a movie, you could record an audition tape, upload it to YouTube, and hope someone finds it and wants to make you the next big star. The chances of that happening are slim to none. You could take it a step further and find the e-mail addresses of every casting agent in your area and send them the link to the video. But the vast majority of them are going to see you going outside the normal process and just delete your e-mail.
What is the downside of trying a move like this? You only have so much time and energy, and you have to choose how to spend it. If you took the time you were spending copying and pasting links into e-mails no one may ever read and instead went to some actual auditions, or set up coffee dates with people who work in the industry, you would have invested that same time much more valuably.
So why bother making your online presence presentable? Because after you go through the recommended channels to seek a job — whether it's through contacts you make via informational interviews or just through applying to a job posting — and you catch someone's attention, you want to make sure that the next things they dig up about you are good.
Your average hiring manager is busy and doesn't have an hour to spend on finding the Internet history of every single candidate. What you want to ensure is that when they Google your name, the first hit is a polished LinkedIn profile that expands upon your resume (and possibly shows some connections you have in common with the hiring manager), and the second hit is a professional website, or a Twitter account that has a record of your engagement in relevant topics.
Use social media wisely, but don't rely on it to land you a job by itself.