Yammer
Yammer is the "Enterprise Social Network", launched in 2008 and sold to Microsoft in 2012. It was originally an internal communication system for Geni (a genealogy website).
Yammer features personal messages, an Inbox of personal messages and announcements from subscribed groups, a files feature and folder, a notes feature and folder (which is wiki-like), an Announcements feature, and a Praise feature. The apps section includes mobile apps and a variety of third-party options.
Yammer stands out from other such social products mainly because of the "Groups" feature that allows you to create & collaborate over a closed topic or group of people & the "Social Graph" feature that allows you to find things easily. O365 have leveraged the social graph feature of Yammer to provide enriched content identification called "Delve". Thus Yammer is not only a stand alone product but also a driver for social in O365.
See Also