Ever since the iPhone and Android turned smartphones into the greatest consumer electronics market in history and helped send Apple's (AAPL) shares into the stratosphere, investors have been hungry for The Next Big Thing®. Something that, if not on par with smartphones, would still see mass-market uptake and yield many billions in annual sales and profits for the market's top players.

For a while, it looked like tablets fit the bill: Shipments of the portable computers saw massive growth in the first years following the iPad's 2010 launch, and briefly overtook PC shipments. But growth turned negative in late 2014 and has remained that way since, as consumers decide phablets are good enough for many computing tasks and that PCs remain indispensable for others. Research firm IDC estimates 183.4 million tablets will ship this year, down 11.5% annually and a small fraction of the 1.46 billion it smartphones it expects to ship.

As tablets fizzled, many began eying wearables as a big opportunity. Fitness tracker leader Fitbit's (FIT)  strong growth in the quarters leading up to and immediately following its June 2015 IPO played a role, as did the hype surrounding the Apple Watch's April 2015 launch.

But the hype began diminishing as sales of both the Apple Watch and rival smartwatches sputtered. And Fitbit's latest results and guidance, released on Wednesday afternoon, might act as the nail in the coffin for hype about wearables, at least until new products and features arrive that fundamentally change how consumers perceive the devices.

Fitbit, estimated by IDC to account for 25.4% of global wearables shipments in Q2, reported Q3 revenue of $503.8 million (up 23% annually) and EPS of $0.19. While the later was in-line, the former missed a $506.9 million consensus analyst estimate. More importantly, the company guided for Q4 revenue of $725 million to $750 million and EPS of $0.14 to $0.18, far below a pre-earnings consensus of $985.1 million and $0.75.

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Fitbit's shares fell by more than a third on Thursday because the numbers. Action camera leader GoPro (GPRO) , which (though competing in a very different market) has been dogged by many of the same concerns as Fitbit about long-term demand, fell 7.1%.

Q4 is easily Fitbit's biggest quarter, and the company is now forecasting its sales will see just 2% to 5% annual growth during that quarter. And tracker shipment growth might be negative after factoring in the impact of higher average selling prices (ASPs) and Fitbit's subscription service growth. Suddenly, the fitness tracker market doesn't look much stronger than the smartwatch market.

Moreover, this growth slowdown comes at a time when a growing proportion of Fitbit's sales are going to existing users. Forty percent of Q3 product activations involved repeat buyers, up from about 33% in Q2.

Given the many reports there have been of fitness tracker users halting their usage after a few months or so, Fitbit likes to trumpet these repeat purchases as evidence of healthy customer satisfaction. That may be true to an extent, but at a time when Fitbit's total tracker sales growth is slowing to a crawl in spite of penetration rates still being fairly low, the fact that a large and growing portion of sales involve existing users doesn't signal good things about broader consumer interest.

TheStreet's Jim Cramer expressed concerns about Fitbit's addressable market earlier on Thursday. "I think that everybody who wants a Fitbit has one," he said. Cramer added he thinks CEO James Park "has lost any credibility" after suggesting in early October that Fitbit demand remained strong by pointing to good sales on Amazon (AMZN) .

The fitness tracker market isn't going to disappear -- many health-conscious consumers find them useful, their low cost and strong battery lives remain selling points relative to smartwatches and they can handle functions such as sleep-tracking and continuous heart rate monitoring that smartphones either can't handle or struggle with.

But it's still ultimately a niche market, just like the smartwatch and action camera markets, and various other consumer electronics markets that have been aggressively hyped over the years.

All of this is worth remembering when gaudy sales forecasts are provided for a hot new consumer technology -- say, home assistants or virtual reality headsets. What the smartphone has accomplished since the iPhone's launch is quite unique, and until there's good evidence to the contrary, it's best not to assume consumers will embrace a newer technology on anything resembling a similar scale.

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