Amazon is about to surpass Costco on one crucial measure

There are now roughly 80 million Amazon Prime members in the US, as of March 2017, according Consumer Intelligence Research Partners estimates. That's a 37.9% increase from the 58 million members Amazon had at this time last year.

Costco, meanwhile, reported in March that it had 88.1 million total card holding members.

For a long time, analysts argued Costco was "Amazon-proof."