If yesterday wasn't all that great for Twitter, today is starting off even worse. The social media company's share price has fallen some 20 percent in pre-market trading, to $51.49. 

The possible culprit? Twitter's "challenging" design could be keeping new users away. Chief executive Dick Costolo told investors yesterday that the company is working on ways to make it easier for new users to sign up and current ones to find the information they're looking for.

We simply need to make Twitter a better Twitter," Costolo said.

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In its fourth quarter earnings report yesterday, Twitter beat financial expectations, posting a small adjusted earnings per share, but user growth numbers were disappointing. Twitter's number of monthly active users grew only 4 percent from the third quarter to 241 million. Twitter was expected to reach 249 million.

Another key performance indicator, called Timeline views, was also below expectations. When compared to the previous quarter, Timeline views for the fourth quarter declined 7 percent to 148 billion. Year over year, Timeline views were up more than 25 percent in the quarter, but growth was slower compared to the a 50 percent increase Twitter reported during the third quarter.

Twitter defines "Timeline views" as the total number of timelines requested when users visit Twitter, refresh a timeline or view search results.

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