Everyone owes it to themselves to spend a day setting up a profile for each site on every major social network if for no other reason than to reserve the name.
During that day sign up with a service like Feedburner that will notify your twitter accounts when you post. Most social networks allow for automated posting and the ones that don't will passively look at your feed from time to time. In other words set it up and forget about it at first while you work on your sites. In time you may see a small following begin, that's when you can nurture it along as netmeg suggests.
Believe me, if the effort you put in on your sites is of superior quality people can and will come looking for you on their social networks. It all starts with your site quality, the networks can't build that for you.