"Post this link on Facebook and get extra features"... would it work? what tools can help implement it?

HI, IH :)
I want to offer my users some extra features if they post the link to my product on Facebook/Twitter, etc.

Have you tried this technique? Tell about your way, please. What results?

Is there any service that can check if the user has really posted my link, and didn't delete it after a while? (if removed, I'll show the alert)