Facebook is losing market share at an even faster than expected rate--and it's probably making Mark Zuckerberg really nervous.

Last year alone, the social network lost more than 1.4 million users in the 12 to 17-year old demographic, according to new report from research firm eMarketer. That represents a decline of nearly 10 percent, or roughly three times what analysts had predicted. Notably, 2017 was the first time that analysts expected the company to see a drop in usage for any age group. Overall, Facebook lost 2.8 million U.S. users under the age of 25, the data found.

The decline may not be terribly surprising, of course. Facebook's "cool" factor has long been eroding, as competitors offer users new, more ephemeral ways to document their day-to-day lives online. EMarketer suggests that teens, in particular, are less interested in maintaining a long-term record of their digital lives, and are increasingly looking to the disappearing messages service Snap, as well as Instagram and its Stories feature, as alternatives.

Unfortunately for Zuck, 2018 isn't shaping up to look any better. EMarketer predicts another 5.6 percent decline in users between 12 and 17 years old, and a 5.8 percent decline for those between 18 and 24. That likely has executives worried about the long-term dominance of the social media platform.