Michael Mace has a very nice post about what you might call "Peak Customers" (by analogy with Peak Oil).

Time passes, and that middle portion of the market gets consumed. Eventually demand growth starts to drop, and you make another price cut. Sales go up again, sometimes a lot. With revenue rising, you and your investors talk proudly about the benefits of reaching the "mainstream" market.

...

What you don't realize at this point is that you're not "reaching the mainstream," you're actually consuming the late adopters. Unfortunately, it's very difficult to tell when you're selling to the late adopters. They don't wear signs. Companies tend to assume that because the adoption curve is drawn as a smooth-sided bell, your demand will tail off at the end as gradually as it built up in the beginning. But that isn't how it works. At the start, you are slowly building up momentum from a base of nothing. That takes years. But by the time you saturate the market you have built up huge sales momentum. You have a strong brand, you have advertising, you have a big distribution channel. You'll gulp through the late adopters really rapidly. The result is that sales continue to grow until they drop suddenly, like a sprinter running off the edge of a cliff.