Netflix may have a challenge keeping the attention of its audience as more rivals release their own original content.

According to data measurement firm Verto Analytics, people are watching Netflix less even as subscriber numbers grow. Average viewing time decreased 13 percent over the last quarter alone — 179 minutes per month in June compared to 206 minutes in March.

Though the data fluctuates month to month, there is an overall decline over the last year. In July 2016, Netflix viewers spent an average of 268 minutes on the platform, and last December it was still at 222 minutes. Verto's data comes from 20,000 of its SmartPanel users, which have installed on-device measurement apps on their phones, tablets and computers.

Still, Netflix is going strong. The company added 5.2 million subscribers, according to its Q2 earnings report on Monday, far above estimates of 3.2 million. (It also posted $2.79 billion in revenue, and earnings per share of 15 cents.)