exclamation. Because shitballs aren't fit to be shouted (or served as appetizers) to all audiences.
Real citation: "HOLY SMURFBALLS MORE BEDARD"
(April 26, 2008, http://www.lookoutlanding.com/2008/4/26/461459/4-26-open-game-thread-part)
Made-up citation: "Holy smurfballs! On my trip to the Twin Cities, I have found bookstores, beer, bros, and a surrogate dog, who I bought some pig ears. Thank Zeus my dog doesn't read the blogs; he'd chew my eyeballs out of jealousy."