Amazon (NASDAQ: AMZN) and MercadoLibre (NASDAQ: MELI) are two titans in the world of e-commerce. Meanwhile, MercadoLibre is at the top of the online-retail space in countries including Brazil and Argentina, and it also has a fast-growing payment processing business. While Amazon's online retail segment still accounts for the large majority of its overall revenue, it's actually its cloud business that is its most important profit generator.