Why does the Pope tweet "du" rather than "sie"?

To add to the list of 'questions your kids ask that you're embarrassed not to be able to answer"  . . .

My son, who is studying German in college, asked me the following, and I couldn't answer.  I don't think it has anything to do with the informality of Twitter, because I think he's right about the informal 'du' being used in religious conversation.   I suspect some MOJ readers know the answer.  Please comment if you do!

I was looking at Pope Benedict's new Twitter account, and out of curiosity, I checked the German one too: https://twitter.com/Pontifex_de. Is it typical to religious authorities like him to refer to people with the informal "you?" Or is that some kind of special "pope" tense? I know that Germans refer to God with "du," so it could be related to that. Or could it have something to do with the Pope being the "father" of the rest of the Church--he uses "du" because he's talking to his "children?"