A customer looks at the new OS X Mountain Lion operating system at an Apple store in Palo Alto, Calif., Wednesday, July 25, 2012. Apple Inc. released its new operating system for Mac computers on Wednesday, with features borrowed from mobile devices and a tighter integration with online file storage. Dubbed Mountain Lion, the new software narrows the gap between the PC and phone software packages, making Mac personal computers work more like iPhones and iPads. (AP Photo/Paul Sakuma) (Paul Sakuma/AP)

Apple says that Mountain Lion, its new operating system for Mac computers, has topped 3 million dow nloads, Bloomberg reports :

Apple Inc. (AAPL) said downloads of its Mountain Lion software exceeded three million in four days, making it the most successful operating system release in the company’s history.

Facebook Inc. (FB)’s social-networking service will be integrated with the software in an upcoming update, Cupertino, California-based Apple said today in a statement. Mountain Lion is available through the Mac App Store as an upgrade to Lion or Snow Leopard users for $19.99.

Apple is adding functions to its software as part of a broader push to make its products work more seamlessly together, encouraging 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 fosters “stickiness” so that users won’t want to switch to another product because of all the time and effort they have invested with Apple’s gadgets, according to Chris Jones, an analyst at market researcher Canalys.

The announcement comes just days after “Web tracking firm Chitika”said that the new software accounted for 3 percent of Mac traffic around the world in just 48 hours after hitting the market, VentureBeat.com reports :

Chitika says that based on the 66 million Mac users Apple claims it has, it can infer 2.1 million Mac owners have already upgraded to Mountain Lion. If 90 percent of those users paid, Mountain Lion has generated $38 million in revenue so far.

Based on these impressive numbers, we wonder if Apple wishes it started offering cheaper and easy-to-download upgrades several OSes ago. Microsoft no doubt took notice of Apple’s successful Lion upgrade option last time, one of the reasons Windows 8 will cost just $40 to upgrade.

So why is Apple’s Mountain Lion such a hit? The Associated Press gives its take on Apple’s newest operating system :

Mountain Lion is made for a world where your computer is just one of your computing devices, along with your iPhone and your iPad. Apple wants to make it easier to switch from one to the other, several times a day.

It’s already easy to switch between iPhone and iPad. For instance, songs and apps you buy on an iPad will automatically pop up on your iPhone through Apple’s iCloud online-storage service. Lion has some iCloud features, but Mountain Lion really brings the Mac into the iPhone-iPad family.

That’s what I like most about Mountain Lion. It borrows a lot from its mobile cousin.

The Mac already had such mobile-like features as the ability to zoom in or out on a MacBook by pinching your fingers on its touchpad. Mountain Lion goes a lot further:

— A notification center slides out from the right of the screen to offer calendar reminders and the latest mail items. It mimics, down to the background color, layout and font, the way you get Facebook updates, news alerts and other notices on your iPhone.

— The Mac’s iChat app has been scrapped in favor of Messages, which is made phone friendly by incorporating the iMessage service for iPhone, iPad and iPod Touch users to exchange texts, photos and video. Now you can send messages from your Mountain Lion computer to your mobile friends, or reach another Mountain Lion user from your phone. The way conversations are presented feels more like texting than instant messaging.

— Mountain Lion borrows a “Share” button from iPhone and iPad apps. The iPhoto image organizer on Lion had that, but it’s now built into other apps such as the Safari Web browser and the Preview document reader. The options change depending on the app. In Safari, for instance, you can send a Web page by email or post a link on Twitter. In Preview, you can share a photo on Flickr or add it to iPhoto.