In April of last year, I started to embrace facebook and twitter as experimentation...to reach out to professional as well as personal contacts. Follow me here.
The result is that I've found both facebook and twitter to be very valuable in sustaining friendly dialogue with friends and new people that I might have only infrequent or very random encounters. Facebook has reconnected me with personal relationships I haven't had in 15 years (or more)...and that's amazing by itself.
As for business relationships, I am very lucky to already have a very loyal client base, but I feel that the addition of social network dialogue (sharing mutual bits of our personal and work lives) has strengthened these relationships to beyond just being a photographer/client experience. I have many new unexpected friendships.
I must say I'm not engaged in social media for the purpose of driving the bottom line, but social media is a useful tool in solidifying and strengthening all of my relationships. If in some cases my friends happen to help decide where marketing dollars are allocated and which photographers are awarded work, then it may indeed help my business (indirectly).
I still have to be the best photographer for the job to be hired...but sometimes an ongoing conversation makes you a natural fit to reach out to when a client needs something new done.