My immediate reaction was “Why? What on earth would you have to share that would matter to most people?”
By the way – this email’s filled with links to their Facebook page, making it super-easy to take action right now.
One caution: don’t promise Facebook-only extras and then make them available elsewhere at the same time.
If you promise special Facebook-only offers, don’t send the same offers at the same time to your newsletter subscribers. They’ll probably notice, and won’t appreciate your bait-and-switch marketing tactics.