Originally, I had thought that when you deactivate your account on Facebook, all traces of your account vanish and do not appear until you reactivate your account. 

But today, I was surprised to see that this is not the case. In fact, Facebook explicitly tells you that your friend has deactivated his/her account:

I’m wondering why Facebook made this product choice…

Is it to prove to you that your friend didn’t de-friend you, and that they simply are taking a leave of absence?

Is it because they want you to reach out to your friend off of Facebook and ask them to come back to the ubiquitous social network? In a way, such messaging may be seen as a watered down scarlet letter on your friend. 

Honestly, I’m not sure I understand why this functionality is designed this way. To me, the more intuitive design would be to not include this friend in your friend list if they have deactivated their account.