https://techcrunch.com/2019/04/15/facebook-hybrid-feed/

Feed and Stories unite! Facebook is so eager to preempt the shift to Stories that it might even let us use the same interface of horizontally swipeable cards to sift through News Feed posts. If users won’t scroll down any more, Facebook’s ad business could take a huge hit. But by allowing traditional feed posts and ads to appear amidst Stories in the same carousel you’re more prone to swipe through, it could squeeze more views and dollars out of that content. This would help Facebook gracefully transition to the post-News Feed era while it teaches advertisers how to use the full-screen Stories ad format.

In this image, you can see a user in mid-swipe through the hybrid carousel between a News Feed story about a friend updating their profile photo to an animated GIF-style video on the left and a Stories video on the right.