Amazon Prime is a vital part of the company's e-commerce business: customers who belong to the $99-per-year loyalty program spend more than twice as much as people who don't, according to several recent surveys.

It's also growing like wildfire. New data from Piper Jaffray shows that half of all households in the United States are Prime members, up from about 35% two years ago. The penetration is highest, and growing fastest, among upper-income households with more than $112,000 a year in annual income — more than 70% of households in that demographic have a subscription, up from less than half two years ago.

(Piper Jaffray)

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