From Facebook Developer Wiki

Please observe the following guidelines for using action links in your Feed stories.

1. An action link is a succinct verb phrase that describes a contextual action.
   • OK: "Become a fan"
   • Not OK: "Are you a fan of Radiohead?"
2. An action link must start with a verb.
3. Each action link can contain only one verb.
   • OK: "Watch the video"
   • Not OK: "Watch the video and win a prize"
4. Action links cannot be deceptive.
5. Action links should not include unnecessary punctuation or formatting characters.
   • Not OK: "Watch the video!!"
   • Not OK: "Watch the video (you will love it)"
   • Not OK: "[ Join Us! ]"
6. As a rule of thumb, action links should be 3 words or less.
7. Do not include calls to action in the Feed story body or headline.
8. A Feed story can contain only one action link with a max of 25 characters.