There's been a lot of concern about Twitter's (NYSE:TWTR) user growth. Indeed, the slower-than-expected growth is probably the most commonly cited reason for the stock's 40% decline during the past twelve months. But how bad, exactly, has Twitter's user growth been? One way to get a better perspective of Twitter's user growth is to compare it with Facebook's (NASDAQ:FB).

Twitter's user growth
For those unfamiliar with growth of the world's largest social media companies' monthly active users, or MAUs, Twitter's recent growth likely appears impressive at first glance. Indeed, even during the company's most recent quarter, MAUs were up 15%, year over year -- and 3% sequentially.

Twitter Monthly Active User Growth

Data for chart retrieved from respective quarterly filings. User data includes SMS Fast Followers. Previous quarters are adjusted to reflect Twitter's recent decision to include SMS Fast Followers. Chart source: Author

But a close look at Twitter's user growth highlights a few red flags.

First of all, total reported MAUs are helped by an inclusion of the company's SMS Fast Followers, or users who access Twitter primarily on feature phones through text messages. With immaterial revenue coming from these users, their inclusion isn't helpful in painting an accurate picture of Twitter's user growth for investors. Excluding the impact of these users, Twitter's users grew 12%, year over year, in Q2 -- and just 0.07% sequentially.

The second major concern is Twitter's growth relative to the size of its user base. While the company's sequential user growth in recent quarters isn't too far off of Facebook's, Facebook boasts a much larger user base: 1.49 billion MAUs. Compare that with Twitter's 316 million MAUs. 

Twitter Monthly Active Users

Data for chart retrieved from respective quarterly filings. Chart source: Author

And not only is Facebook's core platform exhibiting more impressive growth than Twitter's recently, but Facebook's other services: Instagram, Messenger, and WhatsApp -- all services with more than 300 million MAUs -- are growing faster than Twitter, too.

Facebook's user growth
Facebook Monthly Active Users

Data for chart retrieved from respective SEC filings. Chart source: Author

Facebook has demonstrated excellent execution when it comes to user growth. MAU growth remains robust, with sequential growth yet to fall below 3% even as total MAUs nearing 1.5 billion. Even more, the company's daily active users, or users who use the service at least once per day, continue to grow rapidly.

Facebook's DAUs in Q2 were up 3.4% sequentially and 16.8%, year over year.

Facebook Daily Active Users

Chart source: Facebook.

A review of Twitter's growth compared with Facebook's may dishearten some Twitter investors. For a company that wants its social network to appeal to the masses, Twitter's user growth in relation to its size raises doubts about where the social service's addressable market may end. Could the platform be more of a niche product than Facebook and its fast-growing social brands?

While Twitter's user growth is a bit concerning, there's good reason for Twitter investors to keep there heads up. First -- and perhaps most importantly -- the company's co-founder Jack Dorsey is back and he has an aggressive plan to help the service appeal to a larger audience. Second, the company's advertising revenue growth is mind-boggling, leaping 63% during Q2 compared with the year-ago period.

Looking ahead, investors should keep an eye on Twitter's user growth, along with the company's plans to help reinvigorate it. At the same time, investors shouldn't worry too much; Twitter's business is still proving there's plenty of revenue upside, and founder-inspired leadership looks poised to help accelerate some bold changes.

Daniel Sparks has no position in any stocks mentioned. The Motley Fool owns shares of and recommends Facebook and Twitter. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.