Bezos helped create Prime, which started 13 years ago.
Brendan Witcher, a principal analyst at Forrester, said the number of subscribers makes clear that Amazon "keeps laying on more and more reasons for consumers to become Prime members".
"It would make ideal 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.
Additional details about Prime are likely on the way next week - Amazon is scheduled to announce quarterly earnings on April 26. "We know better than that - it's actually happening right now".
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Annual letters from Bezos to shareholders are a popular read in the business world.
Prime gives customers access to free two-day shipping as well as its Prime Video and Music streaming platforms.
There are now 16 Prime countries, with the U.S. the biggest market. In Wednesday's letter, Bezos said Amazon employs more than 560,000 globally. Australia is expected to get Prime later this year. Prime Day 2017 was our biggest global shopping event ever (until surpassed by Cyber Monday), with more new Prime members joining Prime than any other day in our history. Amazon had about 566,000 employees as of the end of 2017, 225,000 more than it reported at the end of 2016.
Amazon has also dramatically grown the number of small businesses and other third-party sellers who compete alongside Amazon's own retail business on the shopping platform.