All you have to do is to to be the first 5,000 followers of Animalloversusa in Twitter.
It is however not clear how they are going to do that since they are not requesting your address.
If it is just a hoax, simply unfollow it.
For more information on the Free Animal Lovers United T-Shirt
Click for US & Canadian residents.
This offer is for residents of USA & Canada.
Is the offer expired? check THIS.