S.H.
hmmm I was gonna suggest Twitter & Facebook, and then was gonna suggest something that cost $$... haha
Could you tell us what type of business it is?
My suggestion was to promotions on FB & Twitter saying that when you reach X likes (or X followers on Twitter) then you will hold a drawing for some sort of prize... people love free stuff! :)
It could be an Amazon gift card, or another type of gift card. Or if your business sells a product or service, you could offer a free product or free or discount service, etc.