People love free stuff. Offer them something, say a gift card, free ebook or any other digital product and they'll be quite eager to help promote your site. If you want a certain number of likes on your FB page for example, someone will at least make an attempt to get those likes so they can get the gift. Wouldn't giving away gifts be expensive especially if you don't want to spend any money? Well it doesn't have to cost you any money. There are many sites where you can find free Ebooks and other free digital products which you can get for yourself and afterwards upload to your site.