First, you share useful information and/or free stuffs for people/followers, build relationship with others users, while you promoting your brand/product/service.
The most effective social media sites right now I think is Facebook, Twitter and Pinterest.
Make an account in twitter, and start following people so they follow you back (to increase followers). and start advertising your site on twitter. the followers you get are only for making good impressions for the upcoming real followers.
or you could also purchase packages for facebook likes/fans or twitter followers in cheap price. I think this is the website that gives best results http://getseosets.org/
There are many ways to utilise this, and it is best to go with a few, Facebook has excellent targeting for ads, far more sophisticated than Google and the sheer user volume should make this a must. Twitter can be very lucrative, Pinterest is growing fast, but also utilise things like Digg, Delicious and Stumbleupon, these can also bring a lot of traffic your way. With each of them, read up and find out exactly what you can do to use them effectively, they can have huge potential...