Sling TV is by far the most popular over-the-top linear TV service. The streaming service from DISH Network (NASDAQ:DISH) launched at the start of 2015. As of the end of 2017, it had over 2.2 million subscribers, over 1 million more than AT&T's (NYSE:T) DIRECTV Now. DISH decided to break out Sling TV subscribers for the first time with its fourth-quarter earnings release.

But despite the strong growth in Sling TV, DISH has seen its overall subscriber count fall over the last three years. In fact, subscriber losses are accelerating as DISH sees a decline in gross additions for its television services. What's more, Sling TV saw subscriber growth slow in 2017 as new competitors including DIRECTV Now, Hulu Live, and Alphabet's YouTube TV entered the market.

On top, but for how long?

DISH saw strong Sling TV growth in 2016. Net additions increased from 526,000 in 2015 to 878,000 in 2016. That trend failed to continue in 2017, as net additions fell to 711,000. DISH's 2017 growth trailed DIRECTV Now, which added 888,000 net new subscribers last year; that came after DIRECTV Now's hangover from its launch promotions in the early months of 2017.

The gap between the two increased in each quarter of the year. DISH started the year adding more Sling TV subscribers than AT&T added to DIRECTV Now. But by the fourth quarter, Sling TV subscriber additions fell short of DIRECTV Now's by more than 200,000 customers:

And AT&T is still being very aggressive with DIRECTV Now. It offers $15 off the service to customers subscribing to one of its unlimited data plans. That lowers the effective starting price to just $20 -- the same as Sling TV's.

Not to mention, Sling TV is also facing challenges from YouTube TV and Hulu Live, which both launched last year. Integrating their existing services and technology gives them an advantage over non-tech-natives like Sling TV and DIRECTV Now.

Management isn't concerned about the slowdown

When asked about slowing subscriber growth for Sling TV on the company's fourth-quarter earnings call, DISH's management mostly brushed it off.

"We like our position," said Sling TV president Warren Schlichting. He went on to note that the company offers fewer promotional offers than some of its competitors, without mentioning AT&T specifically. DISH Network chairman Charlie Ergen added that users may sign up just for a promotional offer -- like a free device -- and then churn off, which is presumably what happened to DIRECTV Now in early 2017.

Ergen also noted that signups can be seasonal with over-the-top subscribers, especially given sporting events like March Madness. But that argument falls apart when looking at Sling's results on a full-year basis, or even comparing Sling's fourth quarter to that of DIRECTV Now.

The management team echoed statements from about 18 months ago, when they noted that new competitors would enter the market. Ergen felt that new over-the-top services would boost the secular growth of services like Sling TV, resulting in more subscribers for everyone. Indeed, the market grew significantly in 2017, but Sling TV won only a relatively small percentage of customers.

Accelerating subscriber losses

It's interesting that DISH decided to break out Sling TV subscriber numbers for the first time just as they're starting to get worse. Meanwhile, losses at its traditional satellite business continue to pile up, remaining elevated from pre-2015 levels as cord-cutting continues to take hold.

Cord-cutting definitely isn't going away, and with growing streaming competition, it's unlikely the trend at Sling TV will change in 2018. Even more over-the-top television options will enter the market this year.

Sling TV helped DISH hold onto subscribers for a couple years, thanks to its first-mover advantage. But investors should expect that number to start falling much more quickly, as customers move from DISH Network and Sling TV to other options.