Costco and Sam's Club have historically been thought to be "Amazon proof" due to the size and quantity of products purchased by their loyal membership base. I have always argued otherwise as Amazon offers similar bulk purchase options and truly provides a thoughtless and frictionless service. Costco and Sam's have recognized that threat and worked to provide shipping via Google Express and buy online, pick-up in-store. Have their efforts paid off?

Yes and no. Cowen recently provided the graphic below that highlights how quickly the Amazon Prime membership base has grown as a percentage of households. The offering of free 2-day shipping is complimented by free streaming of music, TV, movies and books.

When diving down into the overlap of memberships, it looks as though whilst Amazon has grown, the amount of households with memberships from 2 of the 3 (Amazon, Costco and Sam's) have also all grown. The Amazon/Costco combo grew 6.5% whilst the Amazon/Sam's combo grew 3.7%.

So what does this all mean?

Households are now more willing to have a couple of memberships (24.4%) and in some cases 3 memberships (8.3%).
Amazon's impact to Costco is thought to be minimal since Costco continues to steal market share from Sam's Club. With that said, no retailer is Amazon proof when you consider close to 50% of households will have Amazon Prime.
Amazon Prime is the most popular retail membership in the US and provides many benefits outside of free 2nd day shipping.