What are the best social networks for small businesses?

It depends on customers really, how ever Youtube, Facebook, Twitter, Linkedin, Blogs and websites are some of the preferable media for small businesses.

In no order; Facebook, Twitter, foursquare, Google+, YouTube, Yelp, Instagram.

I have found that if used correctly networks such as reddit can get you a lot of organic business growth. However in public view I'd suggest that stick to twitter, pinterest, and of course facebook.

Know where your customers are - you will find the answere :)