It's a fair question!
In fact, as I was writing this newsletter, one of my clients e-mailed me, and put the question very clearly:
"How do we evaluate (what criteria do we use) whether to go with particular social networking sites? This is not going to stop. There are going to be more and more coming out!"
Here's my answer: Fish where the fish are.
In other words, focus on the social networking sites where your target network (clients and colleagues) hangs out.
If they are on Facebook, use Facebook; if they use LinkedIn, use LinkedIn; if they use Twitter, use Twitter. And everybody uses Google, so use Google+ (Yes, using Google+ gives you brownie points when people are searching Google. Read my blog post here to understand why).
This sounds very simple, and it is. But too many people over-complicate it. Don't! It really is as simple as understanding your target network, and hanging out where they hang out.
Just one thing: I deliberately said "target network" rather than "target market", because social media is not just for getting new customers and clients. It's also a place for networking with colleagues: sharing ideas, asking questions, building your expertise, engaging in interesting conversations, and collaborating on projects.