I use social networking websites to promote my own sites. While this method usually will not get you tons of traffic, it will get you some. It can be a great way to get a new website started.
Like all promotion methods, you will be wasting your time if your site is not good. The better your site is, the more people will come. Once you get some traffic to a great site, word of mouth will lead to a growing flow of traffic. In contrast, no matter how much traffic you send to a lame site, you will never build a constant stream of traffic.
The reason social networking can be good is that you will be able to contact a bunch of your friends and other contacts with a few clicks. They will be interested to see your site. Encourage them to share it with their friends.
Remember, do not spam. Spam is often counterproductive, and it can get you banned from these sites.