From Nieman Labs:
There are currently 42 magazine issues listed in the program, called Prime Reading, including familiar names like Sports Illustrated, Vogue, Bloomberg Businessweek, and Consumer Reports. “Every month we choose a selection of top magazine issues for you to borrow as part of your Prime membership,” Amazon said. Users can download up to 10 titles at once.
Originally, I and probably most people joined up when in the process of buying something big the promise of free shipping prompted a 'why not' shrug to their Prime promotion. Years later and the free shipping pays for the membership but, most importantly (in my case anyway), it has worked glue-like in the manner of airline miles to encourage loyalty. The added stuff like book and magazine content and audio books announced earlier this year are increasingly making a tie that won’t ever break.
Several recent news stories put the number of Prime subscribers over the 50mm household mark. It is also estimated that Prime members, who pay $99 per year for the membership, spend roughly double non-members. (Twice) The reports and estimates do tend to feed on themselves because Amazon is very cagey about reporting out actual data. Other recent reports include BusinessInsider and CNN.