In 2017, Amazon bought $3.2 million worth of ads from The Washington Post and spent $4.9 million "related to digital content", for a total of $8.1 million.
The real money, though, is in the buying power these shoppers wield: Prime members reportedly spend an average of $1,300 a year on Amazon, compared with $700 for its customers who are not members. As of the end of September, Levin said Prime had an estimated 90 million US members alone.
The company hasn't previously offered a specific number about its Prime members, instead keeping the size of one of its most important businesses a secret until now.
"It would make flawless sense to us that there would be over 100 million global members, based not only on that number but everything we've seen since then", Levin said.
Bezos said Amazon Prime saw its best year ever in 2017, shipping more than 5 billion items worldwide with more new members signing up than in any previous year - both worldwide and in the U.S. Without knowing how big Prime is or how much it's growing, it's been hard to figure out how successful the program has been.
He added that in 2017, more new members joined Prime than in any other year.
Bezos kicked off the letter by doling out some advice on how to running a business. More than half of the items sold through Amazon now come from third-party sellers.
Much of the letter focused on the importance of high standards, but portions also concentrated on recent achievements like the spread of Amazon Lockers at hundreds of Whole Foods stores or expansion of Prime benefits to countries like Mexico and Singapore. Having "high standards" is the key, he wrote.