Yammer was cool in that you could point normal people at it and they got it -- Facebook for work. My team unleashed it in a large organization and got like 90% of the users signed up without announcing it formally, and had a core user community of 20-25% of the workforce engaged for awhile. They self-organized some cool communities and we had less trouble than we thought that we would. Unfortunately, there were some procurement issues that prevented us from moving forward with it.
The O365 tooling around groups, social via Sharepoint, etc is a half-completed, inscrutable mess. Our IT kids have a 6 page manual for signing into Skype for business. All of their new, cool stuff is either just built on mail or is some sort of Sharepoint skin. The only good thing about it is that we don't need to try to run the thing on-prem anymore.