This is big. Facebook is winning for targeting and CPA at the moment. I believe a big part of this is due to what they've been doing about Instant Articles.
So anything that helps you define the audience better is a winner.
Photo Credit: image from HTSABO
Facebook’s adding two new Custom Audience creation options, with ‘Dwell time’ and ‘Link sharing’ added to the listing for some users. First spotted by The Next Web’s Matt Navarra, the new options provide more ways to hone in your Facebook ads on likely more responsive groups. With ‘Dwell time’, you can create a list of users who ‘have spent time viewing your display adverts on Facebook or Instagram’. This would enable you to re-target those users who’ve (theoretically) looked at your ads but not taken any action.