Also, do you really think Zynga would release a cow that allows you to harvest 9 Farm Cash an hour? This doesn't seem reasonable or possible at all. If it's too good to be true, it definitely is. Many of these scams will lead you into pyramid schemes, and many can even install malware or lead you to malicious sites. Be very careful and double check that the application is the real FarmVille before clicking on any wall post.
Have you seen any other scams we should be aware of? Post the details in the comments!