Facebook have been heavily involved with the tech development of Diem, they developed the Blockchain and I'm quite certain they're the main tech team behind the entire project. It would make no sense if Facebook did not intend to integrate Diem into their ecosystems.
I suspect we will see Diem USD introduced to Facebook initially (including messenger) and the ability to send Diem to friends via both the main app plus messaging app. They may even introduce the ability to send Diem to users who are not "friends" for example, across the marketplace.
It's quite likely that Diem on Facebook and Instagram will further extend commerce capability between users and brands, enabling instant payment for services and goods that are offered within the group of apps.
Of course, it's speculation at this point in time but I personally feel that Diem will open up a lot of opportunities for both Facebook and its users. User's will benefit from instant, low cost transfer of money while Facebook will extend functionality, features and commerce. Exciting!