There's some good sense here, though I for one am guilty of using more than one network.  But I'm guilty on purpose.

There are several schools of thought on this - one being that because of the size of the various social networking tools out there, you are in a strong position to dramatically improve your reach on the Internet if you use multiple sites.  And of course some social sites offer better SEO juice than others.

What's more important though is that you need to have a clear strategy and a clear understanding of where your target market hangs out online and how you will engage with them - and for what purpose.  

So for example, if your target market happens to be Garage Door Professionals (a special niche I think you'll agree), there are in fact 67 groups on LinkedIn featuring people in this profession.  Two of them have over a thousand members.

The point is that you need to do your research, and whilst Facebook is huge and can 'potentially' offer the best returns for your time online, it may well be that your target market will be better served by you in other places online.

Knowing what social networking sites your customers use is now an essential piece of information that you should gather when you meet a prospect for the very first time and get to know them - whatever industry or profession you serve.