Amazon disclosed that it has passed the 100-million-mark in terms of Prime membership 13 years after the program’s launch, a figure that has been guessed at, but never publicly disclosed, CEO Jeff Bezos said in the company’s annual shareholder letter.

The number of U.S. Prime members was pegged at 85 million in 2017 by Consumer Intelligence Research Partners, representing 63% of Amazon’s domestic customers. Last July, CIRP estimated that the average Prime member spends $1,300 annually with Amazon, compared with $700 for non-members.

Bezos said in the shareholder letter that 5 billion items were shipped worldwide to Prime members in 2017, with more people joining in 2017 than in any previous year. He added U.S. Prime members are eligible for free two-day shipping on 100 million items.