There is a link shortener which gives you money for every click! You can sign up here.
When you sign up you just have to go to "tools" -> "mass shrinker" and put there the URL you want to short.
After it you could just share with your friend that URL you obtained.Great, isn't it? You can earn money just sharing your links.
Pay attention, this URL shortener works great with twitter and the biggest part of social networks, but it has some problems with facebook, unluckly.
Good luck with your earning.