The Twitter mobile application is displayed on an Apple iPhone 6s in this photograph taken Feb. 9. (Michael Nagle/Bloomberg)

Twitter shares tanked as the company reported that its user growth came to a halt Wednesday — in fact, it actually lost some of its core users in its latest quarter. In a letter to shareholders, chief executive Jack Dorsey said the dip was temporary. "We saw a decline in monthly active usage in Q4, but we've already seen January monthly actives bounce back to Q3 levels," he wrote.

Still, that was the last thing that investors, already wary about the company's ability to grow, wanted to hear. Twitter's stock — which closed up 4 percent to $14.98 Wednesday — fell more than 10 percent in after-hours trading, immediately after the report was released before rebounding. Shares were still trading down nearly two percent an hour after the report. That extended the company's losses; as of Wednesday's close, shares were already trading down nearly 36 percent from the start of the year.

The company's revenue was in line with analyst expectations of $710 million, as compared to $479.1 million during the same period last year. Twitter reported a loss of $90.2 million, but it still beat estimates for earnings per share, delivering 16 cents per share rather than the expected 12 cents per share.

Twitter forecast that it would make between $595 million and $610 million in revenue for its next quarter, which is Q1. That is significantly lower than what analysts expected.

But Dorsey remained optimistic. "Twitter has always been considered a 'second screen' for what's happening in the world, and we believe we can become the first screen for everything that's happening now," he said.

User growth, however, was the only story that many investors were looking at, and Twitter did not deliver. It reported 320 million monthly active users, up just nine percent from the same period last year -- and essentially the same as it reported in its third-quarter results.

Dorsey, who officially returned as Twitter's chief executive officer in October, has made several moves to make Twitter more accessible to new users who may be afraid to dip a toe in the fast-moving streams of information on the network. These include the rollout of a "Moments" feature that highlights trends on the network and a sort of digest of tweets that appear under the heading "While You Were Away," when users haven't logged into their app for a little while.  Twitter has also made some important cultural changes, in an attempt to shed an association with online harassment.

The company on Tuesday announced it created a special council that pulls in representatives from more than 40 organizations to advise it on how to deal with online abuse and harassment.

Dorsey has shown he's not afraid to alter things that are core to Twitter. On Wednesday, the firm launched an optional feature that places "important" tweets at the top of a user's feed, rather than simply displaying messages in reverse-chronological order, again offering new users a way to make more sense of their feeds.

All these efforts are also attempts to woo advertisers, who have often been stumped by how to reach users who are often scrolling past their carefully crafted tweets and videos.

In a note to shareholders Wednesday, Dorsey said that Twitter will continue to focus on advertising, devoting several sections to describing new advertising products. He also said that more marketers are turning to the network and finding good results.

He also emphasized that while he's committed to keeping Twitter a real-time product, he's also dedicated to making it more accessible.

"We are going to fix the broken windows and confusing parts, like the .@name syntax and @reply rules, that we know inhibit usage and drive people away,” Dorsey said. "Relentlessly refining Twitter will enable more people to get more out of Twitter faster."