Social networking sites are for social networking, and that's how they should be used - not for
SEO. They have the same potential business value as any other form of networking.
AFAICS, from a business POV the three sectors where they are becoming significant are:
1) Entertainment
2) Fashion
3) Health & Beauty
I know at least two small companies in the fashion sector that do get business through their presence on social networking sites. It's worthwhile business but not high-volume, so it's not something to invest hours in maintaining once it's set up. I also know a number of musicians who use Myspace very actively to share and promote their work. A number of charities are also using them quite effectively, mainly for raising awareness.
Currently most users are teenagers who obviously don't have a lot of disposable income. But if my own kids and their Facebook friends are anything to go by, once they have to go out and earn a living they won't abandon social sites - they just won't spend quite so much time on them.
It's definitely a medium to learn, understand and dip your toe into - but not to hang your shirt on. The initial excitement with the medium may fade, and usage may peak and then decline somewhat. But it's not going to go away.