Social networks are ways to get traffic and backlinks. for me I use facebook, twitter, google+, tumblr, stumbleupon, and reddit. to manage networks search google for social network manager tools. you will find many useful tools.
The response to this question relies on upon precisely where you are with your showcasing and with your business.
In case you're fresh out of the box new in your business then I profoundly suggest that you begin off with only one, ideally Facebook, Youtube or Twitter, and figure out how to ace it.
When you are creating numerous leads every day and feel you have that stage secured then by all methods include another couple into your showcasing.