Adopt from feed: Some species can only be acquired through the newsfeed of your Facebook. Check the "Most Recent" link of your Facebook homepage to see if your friends have posted any adoptable species for you to add to your Zoo. You may want to check frequently as adoption links are only valid for an hour, or until someone else clicks the link.
While you're in the game, you'll occasionally be prompted to post feeds for your friends to adopt as well. Each posts asks that a certain number of your friends help you save the species. After saving a certain number of animals, you will get a bonus from the Wildlife Fund!