Time spent in smartphone apps continues to grow, according to comScore’s latest US mobile app report. Americans spent almost 70 hours in smartphone apps, on average, in June (excluding the web browser). Younger users, aged 18- 24, spent more than 90 hours. More apps are also reaching larger audiences: 60 apps had more than 10 million unique visitors in June, up from 44 a year ago, according to comScore.

But attention is still largely concentrated among a user’s top few apps. The average American spends 50% of their app time in their most-used app, and almost 80% in their top three apps, according to comScore.

The big winner, unsurprisingly, is Facebook. It’s a top-three app for almost 80% of users, according to comScore, and number one for almost half of them. (With 126 million US unique visitors in June, it’s the most popular app in the US by a solid margin. It’s also on almost half of users’ smartphone homescreens, according to the report.) And as Facebook goes deeper into mobile video, its share of time spent seems likely to increase.

To be sure, while attention is an important metric, it’s not the only one that matters. From a revenue perspective, it can be more lucrative for a user to spend 30 seconds in the Uber app, or three minutes in the Amazon app, than 30 minutes on Facebook.

Still, as Facebook has built a hugely impressive mobile advertising business, attracting and retaining its users’ attention is crucial. And Facebook has executed this very well.