Image source: Twitter.

Twitter (TWTR) may have incredible brand strength in the social media space, but that hasn't manifested in a meaningful way when it comes to user growth. Quite the contrary. Each quarter, investors scrutinize the company's user figures very carefully, which is one of its most important operating metrics. Investor concerns around user growth are precisely why Twitter brought back Jack Dorsey as CEO, hoping the co-founder can reinvigorate user growth.

Another challenge is that user growth in the important U.S. market, where Twitter's monetization is highest, has slowed significantly. Twitter has also started to include SMS Fast Followers in its user metrics, which are users who access the service exclusively by SMS text messages. These users are predominantly located in emerging markets. Critics argue that this inclusion is merely a way to boost Twitter's user metrics.

As of the most recent quarter (ending Sept. 30, 2015), Twitter had a total of 320 million MAUs, including SMS Fast Followers.

Here's a breakdown of Twitter's user base since 2013.

Quarter

U.S.

International

SMS Fast Follower (Worldwide)

Q1'13

48

156

 

Q2'13

49

169

 

Q3'13

53

179

 

Q4'13

54

187

 

Q1'14

57

198

3

Q2'14

60

211

3

Q3'14

63

221

3

Q4'14

63

225

4

Q1'15

65

236

6

Q2'15

65

239

12

Q3'15

66

241

14

Data source: SEC filings. User figures in millions.

The underlying concern is that Twitter's product is too difficult to use for the average mainstream consumer. Management is keenly aware of this issue and improving the core product is the top priority right now. At the same time, many people visit Twitter or see tweets without being logged in, so Twitter often points out that its "total audience" is much larger than its monthly active user figures convey.