Everyone hopes that scaling become an issue they worry about, but in most cases it isn't something you need to worry about right away. However, when building a platform, we need to ensure that we build on top of a scalable infrastructure from day one. There are two things I think of when talking about scalability. The first is those short term spikes in traffic, you know when TechCrunch writes something nice about you and your traffic sky rockets only to come crashing down to earth a week or so later. The second is long-term scalability when your mobile application gets featured on the front page of the iTunes store and then goes viral.
Both are important, but each is managed differently. If you don't handle the short-term spikes, you may never get to that long-term growth. At the same time, if you ramp your infrastructure too quickly, you'll end up with a bunch of servers you don't use and an empty bank account.