Only about three months after Apple launched its iCloud service, it already boasts of over 85 million users, the company's Senior Vice President and Chief Financial Officer Peter Oppenheimer said during the Q1 2012 financial earning calls on Tuesday.

iCloud debuted along with the iPhone 4S and iOS 5, replacing the company's old MobileMe service. And its growth in the past three months has obviously been explosive. More specifically, its users have exceeded unit sales of Macs, iPods, iPhones, and iPads combined during the same period.

Why? It is because iCloud can seamlessly help iOS and Mac users sync select file types across multiple devices, automatically.

When you sign up, you will find that iCloud allows you to store music, photos, apps, documents, bookmarks, reminders, notes, iBooks, and contacts. Moreover, it works as an email and calendar server.

Compared to the company’s previous subscription-based service MobileMe, iCloud sports richer features. And more importantly, the service is free, which must be a major reason behind its explosive growth.

 We're thrilled with iCloud and the response from customers has been incredible. It was a fundamental shift, recognizing that people had numerous devices and wanted their content in the cloud, Apple CEO Tim Cook said in the earning call. It's not a product, it's a strategy for the next decade.