Last month, McDonald’s created waves by hiring its first-ever head of social media, tapping a PR veteran to the post to tackle customer relations and blogger outreach, plus various marketing efforts. Big charges are afoot yet again for the $22 billion brand.

This morning we learn that the fast-food giant’s marketing chief, Mary Dillion, is leaving, taking the top spot at U.S. Cellular Corp. Dillon has done much to reshape McDonald’s marketing philosophy by engaging more directly with its biggest critics, moms. You may disagree with its findings, but McDonald’s “Moms Quality Correspondents” program has done an effective job at balancing much of the discussion about the nutritional value of McDonald’s meals. This moms-as-correspondents strategy may be McDonald’s biggest marketing success since the Happy Meal.

Dillon’s other big contribution to the world of consumer marketing is too early to judge. Just before her departure, McDonald’s hired Rick Wion as its first ever social media chief, a position that will combine marketing and sales, customer service and public relations. This “Super Size” hire shows real ambition from the Golden Arches and sends the rest of the corporate world a strong indication of what the social media specialists within an organization will have to deal with in the future. Should be interesting to see how the position evolves now that Dillon has left the company.