It could also mean that their recommendation algorithms are highly effective and managing to get people to spend more time on social media. And if "friends" aren't publishing enough, then foreign content will fill that void. Probably other content the user is interested in.

> “percent of time spent viewing content posted by ‘friends’ ” has declined in the past two years

Which does not mean that the time overall has declined. This could even mean that the time itself spent viewing content posted by "friends" did in fact increase, if the percent of time spent on social media increased enough.