My wife, Deanie, uses FaceBook to promote her photography business which mainly focuses on taking family pictures. Unlike most professional photographers, Deanie charges a flat fee and then gives the hi-res digital files to the customer. She also offers prints at her cost, so an 8X10 is something like $2.00 All of this is a big competitive advantage, and because Deanie gives the digital files away (and because she is usually very fast to edit her work), Deanie is able to post some of her client pics on FaceBook (with her clients permission of course) the evening of the shoot or the next day. Her clients will also often post the pics or use them as their profile pictures.
I think this is important in that Deanie is not trying to co-opt her clients social circle (though Pensacola is a relatively small town and Deanie often runs in similar social circles with her clients) or even to pull her clients into her circle. Rather, Deanie is facilitating her clients' desire to show off her product within their social circles. And I don't think this is at all limited to photographers. In fact, I've written a blog post that briefly touches on this phenomenon: http://www.jeffsextonwrites.com/2009/10/are-you-getting-what-you-deserve-from-social-media/
Deanie has picked up a majority of her clients through Word of Mouth and FaceBook.