Viv,
In my personal experience, by far LinkedIn has been the most effective business networking tool outside of a smile and a handshake. I try to be somewhat active in the Q&A section and responsive about forwarding qualified forward requests. I've used it very successfully to research and get my foot in the door to target customers. it's also been helpful in meeting people in global datacenter markets while exploring partnership opportunities.
I'm very selective about my network, even if it is pretty large. I'll only make a connection with someone who I have directly done business with as a client, vendor, colleague, employee, or manager. Speaking of which, you and I have done business together, connect to me: http://www.linkedin.com/in/mhalligan
The rest of the other social networking site (BizNik excluded) really just exist for every type of social networking except for Business social networking. But, my target customer is likely to be far different from yours, so your mileage will vary.
Michael