 Prime Day is usually a big moneymaker for Amazon, drawing in $12.7 billion last year. But a new report from market research firm eMarketer finds that Amazon's share of online sales during this year's Prime Day is expected to get smaller. That's the third year in a row that's happened, and experts like Engadget's Valentina Palladino point to competing online sales. Both Walmart and Target had kind of their rival sales a week earlier.