If you would like to share your app in FaceBook, Twitter or any other social net refer to "Promote" tab, to "Share your app".
Choose network, e.g. FaceBook and click the icon.
Enter your text in the popup and click "Share" button.
Your message will be posted on your Facebook wall (or in the social network choosed) together with your app name, app screenshot and your app preview link.
You can also share your app via email. Click envelope icon:
Enter your friens email and click "Send" button.
He’ll get all the info about your app via email.