Twitter knows advertising, but does it know how to get new users? That will be the question on investors' minds Tuesday when the company reports earnings for the first quarter of 2015.

Once again, Twitter’s advertising business is expected to have had a good quarter. Analyst estimates have the company posting earnings of 4 cents per share with $456.52 million in revenue, which would be an 83 percent hike since the first quarter of 2014. But revenue growth isn't in doubt. User growth is. 

The San Francisco company added a dismal 4 million new users in the fourth quarter of 2014. By comparison, it added 13 million users in the third quarter, 16 million before that and 14 million in the first quarter of 2014. If user growth is once again in the single digits, analysts and investors will start to wonder about Twitter’s long-term potential.

“The most alarming thing would be if their monthly active users actually went down. Everyone knows that it’s going to grow incrementally. The question is: how much?” said Johnny Won, founder of Hyperstop, a tech consultancy firm. “It’s just growing so slowly compared to what it used to be.”

That’s why over the past three months Twitter has made a number of changes that it hopes will attract more users and make it much easier for them to sign up. The company recently has introduced the ability to record and post videos, created instant timelines for new users and set up a new homepage with more information for users who aren't logged in. And now it lets users opt to receive private messages from anyone, instead of only from users whom they follow.

Besides user growth, analysts also will want to know how user engagement is doing. Last quarter, the company's timeline views metric -- which shows how often users check the feed that shows tweets from everyone they follow -- grew by just 0.55 percent quarter to quarter. However, the company has said that its timeline views metric is no longer relevant and that it will no longer report it as a result. It will be interesting to see what, if anything, Twitter will roll out this quarter to replace the metric and give investors a sense of how engaged its users are.

“They need to give us a new metric. If they don’t like the timeline view metric that they provided as the best measure of engagement, they need to provide a different one and show how that’s a better measure of engagement and show that it’s indeed increasing and improving, because on the last few earnings calls they’ve been telling one story, and yet all the metrics they’ve been showing have been telling a very different story,” said Jan Dawson, chief analyst at Jackdaw Research.

Analysts and investors are confident in Twitter's ability to make money off the users it already has, but if the company can't start to show that it is also keeping those users engaged and adding new ones, Wall Street may sour on the company and its long-term potential.