Facebook said it would take awhile to integrate Apple's 3D Touch features on the iPhone 6S into its apps, and it certainly has. You've been able to peek and pop within the main Facebook app for some time, but only today — two months before the next iPhone — is the same functionality coming to Facebook Messenger.
The chat app now takes advantage of the pressure-sensitive 6S screen, letting you preview conversations, photos, GIFs, videos, stickers, links, and more. And no, this isn't a way around Messenger's can't-be-turned-off read receipts; the other person will still see one even if all you do is peek into a conversation and never fully pop it open. Oh well.
Like many other examples of 3D Touch we've seen, this won't really alter the way you use Messenger. It's not particularly cool, but could make things slightly faster if you even remember the feature's there after a week. I do find myself using it regularly in Instagram, at least.