Downloads of Apple Inc. (AAPL) Mountain Lion software exceeds three million in only four days, making it the most successful operating system ever released in the company's history.

Social-networking service, Facebook Inc. (FB) service will be intergrated with the software in an upcoming update.

Mountain Lion is available through the Mac App Store as an upgrade to Lion or Snow Leopard users for $19.99.

Apple has added functions to its new software as part of a broader push to make its products work more flawlessly together, inspiring customers to purchase multiple devices. Mountain Lion has many features from the iPhone and iPad, including a notification system so text messages sent to those devices will also show up on a Mac computer.

That nurtures a type of technological adhesiveness so that users won't want to switch to another product due to of all the time and effort they have invested with Apple's devices, according to Chris Jones, an analyst at market researcher Canalys.