Twitter has failed to grow its monthly active user count at all in the final three months of 2015, resulting in a sharp fall of its shares.

It had 320 million monthly users on average through the quarter, exactly the same as it had in the previous quarter, according to its latest earnings report.

 In comparison, rival social network Facebook has over 1.5 billion users and recently reported record financial results of their own.

To make matters worse: if you exclude the SMS-only users that Twitter began counting recently, the social network's user count actually declined slightly to 305 million in the holiday quarter from 307 million in the quarter prior.

Twitter appeared to pin the poor user numbers in part on seasonal weakness over the holidays and a decision to send fewer emails asking dormant users to return.

The stalled user growth has led Twitter to re-think many of its fundamentals, including rumors of doing away with its character limit and this week introducing a partially algorithmic timeline.

"We think there's a lot of opportunity in our product to fix some broken windows that we know are inhibiting growth," Jack Dorsey, Twitter's CEO and cofounder, said in a conference call with analysts.

In its earnings report yesterday, Twitter reported it may also "fix" the way people use its iconic @-mention feature, which it says "inhibit usage and drive people away."