Ask HN: startup idea

I was reading an article a few days ago, which quantified how much money is worth for a business when some of their users use Facebook's Like button to share the business with their friends. Suppose you're selling something (movie tickets/rentals, gadgets, etc...) and allow your customers to share/like on facebook what they purchased. Naturally, some of their friends will check it out and even make the same purchase (especially for certain type of products, like events, movies, etc... where it's important if some of your friends attend). I'm wondering what you think about providing this as a service to everyone selling something out there? I'm thinking something like an ad network which enable store owners to let their customers engage with their friends, which in turn drives more traffic/sales to the store owner. The store owner would pay a CPC or CPA fee. Thanks!