Apple's 500 million user accounts are only second to Facebook, but
generate more revenue per account than any other company according to a
research report from Morgan Stanley analyst Katy Huberty.

However,
Apple has nearly 500 million accounts with a credit card on file,
allowing them to tailor its software and services to their users that
competitors can't.

Apple generates nearly $329 per user, while
Amazon (200 million users) is a close second at $305 and finally, eBay
(112 million users) is third with $125 per user.

By years end,
Apple could be looking at 600 million accounts, suggesting new services
such as mobile payments, streaming radio and targeted advertising could
crucial to the company's growth.

via Apple Insider