We all knew that Apple’s new OS, Mountain Lion, was going to be popular, but we didn’t know exactly how popular. With stricter system requirements, it was going to be interesting to see how well Mountain Lion performed in its first week. Turns out it performed better than anybody expected.

Apple announced that Moutain Lion had been downloaded more than three million times in only four days. Such a massive initial showing puts it as the most successful Mac OS X launch in the company’s history. Apple attributes the strong showing to the massive amount of new features that Mountain Lion introduced.

As you probably know by now, Mountain Lion contains over 200 new features. Many of these features go a long way to making Mac OS X more user friendly and more social. The built-in integration of Facebook joins the built-in integration of Twitter to make OS X the most social centric OS to date.

Apple was also quick to point out that the success of Mountain Lion can also be attributed to the Mac App Store. Many users upgraded because they wanted to take advantage of the new features in Mountain Lion with their pre-existing apps alongside new apps that are being added all the time. Couple that with synchronous gameplay across all Apple devices and you have an app heavy machine that Mac fans are going to go nuts over.

As more people buy the new MacBook Pros and MacBook Airs, it will be interesting to see how well Mountain Lion performs over the next few months. Macs are still immensely popular and Mountain Lion is going to be a must for all new Mac buyers. I wouldn’t be surprised if Apple announced well over 50 million downloads of Mountain Lion before the end of the year.