Disney's (NYSE:DIS) streaming service got off to a stellar start on day one, with 10 million sign-ups.

It carried that momentum through the first month following its launch, garnering around 28 million downloads, according to data from SensorTower. Another 31 days later, SensorTower says smartphone users have downloaded the Disney+ app 41 million times, counting an additional 13 million users.

While that might sound like a steep slowdown, SensorTower compares the 33% of total downloads for Disney+ in month two to AT&T's (NYSE:T) HBO Now app, which saw 29% of its downloads through the first two months following its launch come in the second half of the period.

Disney+'s momentum was fueled by the popularity of The Mandalorian after initial sign-ups brought in Disney fans attracted to its back catalog of film and television titles. But now that the first season has ended, can Disney maintain the momentum in months 3 to 12?

The homescreen of Disney+ displayed on a television.

IMAGE SOURCE: WALT DISNEY.

The one number more important than downloads

Alongside its download estimates, SensorTower provided in-app purchase estimates. Notably, the total amount users spent in the Disney+ app declined after the first month, from $53.3 million to $43.9 million. That's interesting because subscription video services generally see increasing month-over-month revenue growth. The decline in revenue may indicate low subscriber retention.

It's worth noting, to demonstrate how big of a success Disney+ has been, that Disney+'s revenue through its first two months was four times as much as HBO Now's in its first two months. That's despite the latter charging a subscription price twice as much as Disney+.

There are a few things that may be going on with Disney+'s decline in in-app revenue.

Disney offers discounted annual subscriptions for Disney+, and consumers that signed up on day one were probably more likely to take the offer than those who signed up later. That's because those users were signing up for a known product -- the back catalog -- while later users may be more attracted to Disney's new originals.

Additionally, the app isn't the only way to pay for Disney+. Consumers can also sign up directly with Disney or through their connected TV device. SensorTower doesn't track that spending. It's possible more consumers discovered Disney+ through their connected TV devices in month two compared to month one, and downloaded the app after subscribing. Disney spent a lot on advertising on those platforms.

Investors won't know for certain what's affecting the numbers most, unless Disney discloses metrics around where consumers are signing up and whether they're taking the annual plan or paying month-to-month.

Over time, however, investors should expect updates from companies like SensorTower to show increases in in-app purchases every month as Disney's subscribe count increases.

Will subscribers stick around?

Disney+ finished its eight-episode run of The Mandalorian at the end of December, and now faces its first real challenge in retaining subscribers. While other Disney+ exclusives have received good reviews, none have captured the attention of the public like the Star Wars saga.

Since launching HBO Now, the premium cable network has faced a challenge to retain subscribers nearly every summer after each season of Game of Thrones. And with the series ending last year, AT&T is expanding the service to include HBO Max as a means to improve retention.

If SensorTower's revenue numbers are an indication that Disney+ app downloaders paid for an annual subscription, then Disney has a long time to find new series and films to keep its audience engaged throughout the year.

At the same time, the step down in in-app purchases in month two could indicate more month-to-month subscribers in Disney+ second month who may have signed up just to watch The Mandalorian. Those subscribers will be much more difficult to retain in month three and beyond.

Disney's first Marvel series for Disney+ won't debut until late 2020, and season two of The Mandalorian is slated for about the same time frame. There's a lot of time for month-to-month subscribers to drop out before then. And new competitors entering the market over the next few months, including HBO Max, will do their best to draw Disney subscribers away from the service.

Overall, SensorTower's numbers show Disney+ has kept up its strong momentum. While the declining in-app revenue is notable, it's not a big red flag so much as a yellow flag warranting a dive into the details management provides in its first quarter earnings report next month.

Disney+ is sure to be the focus of the media company's conference call, and investors should get a lot more detail on its progress in just a few weeks.