The term "cloud" is just another term used to describe the delivery of technology over the Internet (the term comes from network diagrams that used a cloud to represent the Internet). Just as the Internet transformed the way we send messages and information, it is now transforming the way technology providers deliver capabilities to customers.
When was the last time you went as a consumer to a software store, bought a disk, took it home, and installed it on your computer or mobile device? If you've done this recently, savor the memory because this mode of delivering technology on physical disks is rapidly going extinct.
We now shop for apps on our smart phones, click, and seconds later we have a new capability. The consumer market has transitioned almost entirely to the "cloud." Why? The advantages are easy to understand. It's faster, more reliable, and costs far less. (Just compare the cost of apps at 99 cents a piece to the packaged software you bought at a now-defunct Egghead Software or Comp USA store 15 years ago.)
However, the greatest advantage of the cloud delivery model is one that is not immediately obvious: speed. Technology can now evolve and change and be delivered to end-users in minutes instead of months.
Facebook is an enormously complex application, built by thousands of engineers, connecting more than a billion users in dozens of languages in every corner of the globe. Even at this massive scale, Facebook has the ability to roll out new features every week or even every day.