Prime is the gift that keeps on giving for Amazon. According to a new study from Consumer Intelligence Research Partners, Amazon’s premium subscription service has now reached 80 million members in the US. As this chart from Statista shows, that’s about double the estimated amount Amazon had just two years ago.

Now, it’s hard to say exactly how accurate this is; Amazon famously does not give out exact figures for how many Prime subscribers it has. Plus, CIRP even notes that some of its total are either using a free trial or a discounted student (or any other) membership.

But it seems clear that the service is at least growing. That’s a very good thing for Amazon; the barrage of Amazon-related perks it packs into each subscription makes Prime users much more likely to spend time on Amazon's site and to buy more things. According to CIRP, the average Prime member spends about $1,300 on Amazon per year, nearly double the estimated $700 spent by non-Prime members. That’s on top of the $99 a year (or $10.99 a month) each one pays for a subscription in the first place.

Add all that to its increasingly large AWS cloud business for corporations, and a still-growing online shopping industry, and you can see why some analysts think Amazon will be one of the first trillion dollar companies.