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Fitbit (FIT - Get Report) slumped 14% to $5.91 in premarket trading Thursday after the wearable devices maker delivered weak guidance for the first quarter.

On Wednesday, Fitbit posted adjusted earnings in the fourth quarter of 14 cents a share, a reversal from a year-earlier adjusted loss of 2 cents. Revenue in the quarter was $571.2 million vs. $570.8 million a year earlier. Analysts expected Fitbit to earn an adjusted 7 cents a share on revenue of $569 million in the fourth quarter.

Fitbit said it expects an adjusted loss in the first quarter of 22 cents to 24 cents a share on revenue of $250 million to $268 million. Analysts were calling for a loss of 15 cents a share on revenue of $272 million. For the full year, Fitbit said it expects revenue of $1.52 billion to $1.58 billion vs. estimates of $1.57 billion.

Fitbit said it sold 5.6 million devices in the fourth quarter, topping analysts' forecasts of 5 million.

"I'm proud of our performance this year - our results demonstrate that our strategy is the right one, placing us on a path back to growth and profitability," said James Park, co-founder and CEO. "We grew our active users 9% to 27.6 million, became the No. 2 player in the smartwatch category in the U.S., and grew the number of devices sold in the fourth quarter.

"In 2019 we're committed to offering more affordable devices with engaging health and fitness features, making the health benefits of being on Fitbit even more accessible. As a result, we are forecasting active users, devices sold, and revenue to grow in 2019," Park added.