Aunt BBC has not got it quite correctly. I live in Germany, and my impression is that "Shitstorm" is used as a technical term for "public outcry at Facebook and other Internet media". Obviously the word was not heard, but read by German speakers on such an occasion, and adopted for the specific meaning. Such technical usages are normally short-lived.

In dialects of northern Germany, where Merkel was born and raised, the word "Schiet" is used for "mud" (as in Mudcat), not vulgar at all. So rather than batting an eyelid, she rightly thought herself speaking in vogue for young voters. However, not all of these may appreciate a politican using their jargon.