Social commerce sites like Gilt, Rue La La, ShoeDazzle, Groupon and LivingSocial have been among the leaders in 'social marketing'... and they find success through relatively straightforward, sometimes simple products and promotions. Here is a great example: Gilt is running a promotion where users who invite 10 friends to the service get free shipping. Clear value proposition and very simple process to invite. Furthermore, it is an attainable result (not tied to purchases and not a crazy number of invites).
And of course the site / brand itself is 'share-worthy' - if Gilt provided a poor experience, these sorts of promotions wouldn't result in sharing or ROI.
"Got 10 Friends? Get Free Shipping" "Invite 10 friends to Gilt and enjoy free shipping on your next order. Spreading the word is always in style."