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 ever-expanding 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.

Although providing free two-day shipping to Prime orders obviously drives up Amazon�s costs, the service so far has proven to be worth the expense. Ives estimates that Prime members typically spend twice as much at Amazon than non-Prime members.

The service also helps spur sales of Amazon�s internet-connect speaker with its digital assistant, Alexa. The device makes it easier to shop online and gives the company a toehold in homes.

Amazon also is leveraging Prime to spur more sales at Whole Foods, the brick-and-mortar supermarket chain that it acquired last year, by offering two-hour delivery of groceries to the service�s subscribers.

Prime serves as a �competitive moat� that makes it more difficult for other retailers, online and offline, to get people to shop at their stores instead of Amazon, Ives said.

�It has become a golden ticket for Amazon,� he said.

All those factors have helped make give Amazon a market value about $740 billion. Only Apple, at about $900 billion, and Google�s corporate parent, at about $745 billion, are currently worth more.

President Donald Trump has recently attacked Amazon as an economic scourge, deriding the company as a tax dodger and an abusive customer of the U.S. Postal Service. That has raised worries the Trump administration might take action that could crimp Amazon�s growth, but the growing popularity of the Prime service could make that a politically unpopular move.

(AP)