However, Fitbit seems to believe this quarter won’t be nearly as good, suggesting revenues will come in between $420 million and $440 million, which is about $50 million less than consensus estimates. That disconnect caused Fitbit shares to sink more than 15 percent in after hours trading. Fitbit says its leaning on the conservative side as it spends on manufacturing and marketing for its recently-announced Blaze and Alta wearables, which are expected to ship next month.

Fitbit's active user base grew, but the criteria for 'active' is pretty light

Fitbit also shared some interesting stats around engagement, or, people who are buying Fitbits and then actually wearing them, not tossing them into a drawer. It's a topic we've written about before, and Fitbit has said that it is only going to share these numbers annually, not quarterly. Fitbit says its total "active" users grew over 150 percent to 16.9 million by the end of 2015, up from 6.7 million active users the year before. Of 18 million new Fitbit users in 2015, 72 percent were still considered "active" by the end of the year.

So Fitbit, the market leader for activity trackers in the U.S., had a great holiday season — no surprise there, even with the introduction of Apple Watch in 2015. But it’s the next couple of quarters that are going to be critical for Fitbit, both with the launch of two new products and with more and more competing wearables coming down the pipeline. Both the Blaze and the Alta have their activity-tracking work cut out for them.