Fitbit released its fourth quarter earnings report on Monday afternoon, beating analysts' expectations in terms of revenue, and nearly doubling its full-year unit sales from 2014.

The San Francisco-based maker of digital health and fitness products raked in $712 million in revenue last quarter, significantly beating analysts' estimates. That's a 92 percent jump up in year-over-year from revenue for the quarter. The company, in its earnings call, credited much of this to a strong 2015 holiday season, especially in the U.S.

In terms of unit sales, Fitbit sold 8.2 million activity trackers in Q4 2015, up from 5.3 million unit sales the previous quarter. Its yearly unit sales nearly doubled, to 21.4 million units sold in 2015 from 11 million in 2014.

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.

However, the total number of registered Fitbit users by the end of 2015 was 29 million, which means just around 58 percent (16.9 million) were considered active. And the criteria for being active is pretty light: Fitbit defines active users as anyone who has an active subscription to Fitbit's premium services, who have paired a tracker or scale to a Fitbit account, or who have logged at least 100 steps or taken a weight measurement in the past three months.

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.