Amazon now has 100M members

SAN FRANCISCO -- Amazon has persuaded more than 100 million shoppers to subscribe to its Prime service that offers free two-day shipping and other perks that help bind people to the company and its everexpanding empire.

CEO Jeff Bezos quantified the size of Amazon's Prime membership for the first time Wednesday in his annual letter to the Seattle company's shareholders. Before Bezos' revelation, analysts had been left to guess how many people had been willing to pay $99 per year for the Prime service, which Amazon launched 13 years ago as a way to foster customer loyalty.

The scope of Prime's success stunned even the most optimistic of analysts, such as GBH Insights' Daniel Ives. He had previously estimated Amazon had 92 million Prime subscribers.

"It's a mind-boggling number that serves as a key barometer to how big Amazon's kingdom has become," Ives said.

Costco has been selling annual memberships to get bargains on goods stocked in its warehouses for decades, and already been outdone by Amazon's Prime service. Costco has 50.4 million memberships that allow 92 million cardholders to shop in its warehouse and website.

Amazon still isn't the king of digital subscriptions, though. Netflix has 125 million worldwide subscribers who, on average, pay about $120 annually to watch its video-streaming service, based on numbers the company released with its quarterly earnings earlier this week.