Despite competition from cheaper tablet computers such as Amazon.com Inc.'s Kindle Fire, the iPad remains the most popular tablet computer. Apple Inc. has sold more than 55 million iPads since its debut in 2010. For some customers, standing in line was the only chance to get a new iPad on Friday. Apple quickly ran out of supplies it set aside for advance orders. The company was telling customers Thursday to expect a two- to three-week wait for orders placed through its online stores. Some buyers feared even longer waits.