Here's a great chart from BI Intelligence looking at one of
It has ~600 million accounts on file, most of which are linked to credit cards, which is light years ahead of its competition according to BI Intelligence.
The 600 million accounts is really astounding when you consider that for people to get an account with Apple, they likely had to spend at minimum $200 for some sort of iPod, iPhone, or iPad.
Free, popular web services like Twitter don't have 600 million users. Yahoo, which is one of the most popular websites in the world has ~700 million monthly users. Apple isn't that far from Yahoo.
The bigger picture here, as far as Apple is concerned, is that the company has a healthy user base. Even if Apple totally botches execution of new products, it's going to be around for a long time with this many users.
Business Insider
Disclosure: Jeff Bezos is an investor in Business Insider through his personal investment company Bezos Expeditions.