Asymco's Horace Dediu points out that, on average, over half a million iTunes accounts are being created each day. At WWDC this week, Tim Cook announced iTunes has 575 million accounts. Horace Dediu took that figure and calculated it with previous iTunes membership milestones to find the average number of new iTunes accounts created daily.
If Dediu's trend continues, iTunes will add another 100 million account holders this year. If it holds for three years, iTunes will have more account holders than Facebook by 2017. That is, of course, only if Facebook doesn't continue to add accounts, which it probably will. These numbers still demonstrate just how large iTunes is. If iTunes were an actual social network, it would be second only to Facebook and larger and Twitter and LinkedIn.