Just got this email:
Microsoft Studios is always evaluating its portfolio of products to determine what is best for gamers, families and the company. This decision is the result of the natural ebb and flow of our portfolio management.
Customers who have already downloaded the Windows 8.1 version will still be able to play Zoo Tycoon Friends and use any content they have already acquired, but effective December 10, 2014, Zoo Tycoon Friends is no longer available in the Store.
On April 28, 2015 we will be discontinuing the Zoo Tycoon Friends service. At that time, the game will cease working. We will continue to provide community support for the title through that date. Thank you for your interest in Zoo Tycoon Friends.