Iskra- great question.
Social networking simply needs a target audience. This first means that you have to have a purpose to networking: it doesn't matter if it is real life or online.
As a business: I have directly gained multiple clients from different sites, including Biznik, Linkedin, Facebook, Wordpress, ActiveRain, FastCompany, MyRagan, and Intuit.
I have also found through social networking: half a dozen reseller partners, at least three vendors, an investor, an employee, about ten freelancers, and numerous media contacts... along with roughly fifty professionals that I refer work to (give and receive.)
I also do a lot of research when I am networking. Before most Biznik events I have usually looked at EVERY attendees profile to ask four questions:
1- What can I do for them?
2- What can they do for me?
3- Who can I introduce them to?
4- Who can they introduce me to?
In my opinion, to ask "is there any business here?" is not really a sustainable method of networking. Business networking is a very karmic method of give and receive.
Another faulty mindset in business networking is typically thinking now vs future. Depending on the scale of what you are doing, many arrangements take great amounts of time. My typical contact requires ninety days to network with. If I am looking for an enterprise contract it is usually 6 to 9 months of relationship building.
Social networking tools do not accelerate the growth of relationship. It does expose you to a better selection of prospective candidates that you must choose to cultivate over time (and planting the wrong seeds now will still produce little or no return.)
With this in mind, I suggest anyone looking at these tools to really examine who they are reaching out to and why. Having a three and six month plan is essential, as you move people from introductions to friends.
In the end, it is all about who you select to be in your inner business circle.