All of the social networking sites need to be used as reference points and points of client service to maximize their effectiveness.  If you rely on them for new business, then you'll be disappointed.  Offering them to your current customers as a mechanism by which to stay in contact with you and for you to keep them abreast of industry trends is good.  Informing them about special offers, seasonal discounts, and other opportunities is also nice.  It is a part of long-term customer care and relations. 

It can also help to separate the fly-by-night posers from the long-term industry professionals who have an established presence on the net.

It can be difficult to provide personal service touches to projectes ordered by firms through email who are many states away.  Offering a link to social sites establishes online community, which is a good thing in our digital age.