FB Work: Groups help you make decisions and keep your team on the same page.
Yammer: Organize all your team communication in one place with Yammer Groups. Create a single destination for your messages, files, and updates, where everyone has a view of what's going on without all the usual back-and-forth.
FB Work: Get through to the people you need right away with Work Chat.
Yammer: Spend less time crafting perfect emails, and more time doing. Conversations in Yammer make it easy to quickly share information on a topic, have active discussions around it, and agree on next steps.
FB Work: Find the answers, past group posts or the files you need using search.
Yammer: Find what you need, right when you need it. Yammer seamlessly enables connections to people and information from across your organization that you never knew existed, making it easier to move your work forward.
FB Work: As a Facebook at Work customer, you also have controls to manage your organization’s data how you see fit.
Yammer: Public Groups for working in the open; Private Groups for sensitive ideas. Safely collaborate with partners and customers with External Groups.
Yep. Yammer.