What's a good service to use for visitor referrals, i.e, a contest-system that gets visitors to your site in exchange for "entries".

I had a look at GetAmbassador, but the pricing is a bit hefty for a content site that's not selling a product or service. The site is monetized by way of 3rd party sponsors, which we're looking to acquire more of. Currently I am using ShortStack, but I'd prefer to have something that integrates within the site itself rather than run the promos through Facebook. I basically need a way for site visitors to easily share my website with their friends, then in return, the users who send me visitors that turn into mailing list subscribers will receive one entry into a monthly prize giveaway. Does anyone here know of such a plugin (WordPress) or better yet, 3rd party service which does this - at a reasonable startup rate? :) Thanks in advance!