This setup probably makes people feel like they're being bought. You can still buy their clicks, but you'll probably have to figure out a way to make them feel like it's all their idea.
Overall, what everyone is telling you (here and in the comments on your blog post) is that putting links and/or endorsements on a social networking service is much more than just a fee-for-service proposition; there are other intangible qualities that are important, primary among them what your friends on these social networking services think of you. You want to believe that your friends post links to things because they're good, not because they're getting a discount. In fact I'd argue that one of the main reasons people like social media in the first place is that it's a person-to-person interaction that's not commerical in nature.