With my previous employer we had set up a Facebook page and a blog at our website. I see Facebook for the business side of things as a potentially useful tool, if you can get your friends to "Like" your page. Another possibility is to just occasionally post status updates on your personal page looking for work. You never know which random "friend" you haven't talked to in 3 years will know someone who needs some work.
As for blogging, it can also be quite beneficial, but it takes a lot of work. You really need to keep up with it, writing 2-3 interesting posts per week. It
can be a great tool to bring new business in though. A large factor in search engine ranking is links from other websites to yours, so if you're writing meaningful content that people like and link to it can seriously affect your search engine rankings. Again, this requires constant, interesting posting however, which is a lot of work.
Twitter's basically the same concept as the blogs, but again is a lot of work to maintain and you have to be "interesting" in significantly less space.
That said, you can link all of these things up with the right plugins, and do something like any time you post to your blog it will automatically post to Twitter and update your Facebook status.
So basically what it comes down to is: all of these can be useful, but you really need to put in the time and effort to make it work.
A great example of someone who does this, and talks about how to make it work, is Gary Vaynerchuck. Check out some of his videos at
http://www.viddler.com/explore/garyvaynerchuk/videos/ He's incredibly passionate about using social media to build a business, and many of his thoughts apply very well to the Freelancing world.
NOTE: Not all of his videos are entirely safe for work if I remember correctly.