McDonald's said Tuesday a key sales figure declined 3.3 percent in the U.S., marking the fourth straight quarter of declines for the world's biggest hamburger chain. McDonald's (14,000 U.S. locations) is far bigger than Chipotle (1,700 locations), making it more difficult for the chain to boost sales. To defend the image of its food, McDonald's launched a social media campaign last week inviting customers to ask questions about the ingredients it uses. The company has run similar programs in Canada and Australia, where it says the program has built trust with customers. McDonald's executives have said they introduced too many items too quickly last year, such as McWraps and the option to substitute egg whites for its breakfast sandwiches.