Sign up to such networks as appear sensible to you - you can't participate in them all, but some definitely service different markets (e.g. LinkedIn and Facebook are entirely different).

Whichever ones you sign up to, be sure to leverage the following:

Build reciprocal links to your website

Build a "Page" in that social engine about your business

Use multi-media

Suggest your page to your network

Participate in the network.  Post open-ended questions rather than offer closed-comment answers all the time

Make use of the engine's "share" facilities

Use their feeds to embed back into your website as newsfeeds

Obtain "Likes" and "Diggs" from your friends

Include social network addresses in your advertising