Dive Brief:

• Amazon has quietly shifted its Prime membership rules to limit sharing of a membership to two adults in one household, Quartz reports.

• Just recently, up to four adults in one household could share a Prime membership.

• The change wasn't announced; customers noted it and Quartz discovered it via various social media reports.

Dive Insight:

Amazon’s tightening up of its Prime membership rules is a reflection of the maturity of the Prime program—the retailer needs more customers and the pool of Americans who aren’t already members is shrinking. 

The change will likely affect many college-age students or young adults still living at home (or sharing their parents’ account even if they aren’t technically at home).

Lately, and from now on, changes and innovations from Amazon are likely to boost Prime membership or have advantages for Prime members only, which will only once again boost Prime membership. (See: Prime Day.) And while allowing Prime members to share more liberally at one time was a great way to spread the word, Amazon now needs those hangers-on to be paying members in their own right.

The e-retail giant is laser-focused on Prime for good reason: those customers are really sticky. Amazon Prime members convert 74% of the time on Amazon.com, according to a study from Millward Brown Digital, compared to 13% for non-prime members. And Prime members have a preference for Amazon, converting 6% of the time at other warehouse retailers’ e-commerce sites. 

There are now some 44 million Prime members, half of Amazon’s customer base, and if membership growth continues at its current pace, half of all U.S. households could be Amazon Prime members by 2020, according to Millward Brown (Estimates have to be done by outside bean counters; the retailer doesn’t reveal them itself.)