There are not enough hours in the day to successfully manage a plethora of social media profiles.  The bottom line is that you need to make money.  Social networking sites will help you build a strong network of contacts, but you still must devote a large portion of your business day to selling and perfecting the actual services you provide in order to generate some sort of income.  Personally, I have taken a liking to Facebook and Twitter.  I never managed to build up a strong enough Digg reputation and StumbleUpon is not a site I travel to very often.