Sites like Twitter and Facebook are great places to promote your website. On Facebook you can create groups centered on your topic, and network with other users who have an interest in the topic of your website.
On Facebook you can create a fan page, so you can use that to promote your website as well. Founders of Twitter don't classify it has social networking site. I would say it falls in that category. Alot of people think Twitter is a waste of time, but if you learn how to use it, it can be a great way to generate traffic. Great Networking tool.
Here is an example. Do a video explaining how to use a certain product. After your done send out a tweet to the product owner. Soon enough you will receive a tweet back from the developers of the product, thanking you for free promoting there product. They will not only thank you for making a video tutorial of your product but create more traffic to your website.
Social Networking sites can be great promotion tools, but just like anything you have to use them effectively to get results. That may mean you have to start testing and experiment for a while to see what works best for you.
I do spend a lot of time marketing on social networking sites. Social networking is not for everyone and you can still have success without using social networking sites to promote your website.