We use Yammer and it has been a struggle to get people to use it. However, that is partly our fault for not publicizing it more. We have some groups on campus that have really embraced it and have positive things to say about it, such as researchers who share resources. We also have a program called OneBook, which is an annual book club that holds its discussions in a Yammer group. Those go pretty well. When departments or units request a social media presence that doesn't fit our external audiences, I encourage Yammer, but there has been some resistance from people who want a more traditional external option. They nearly always select a closed Facebook group.
My husband works in a different industry and Slack is what they use internally. There are very lively conversations on there that we don't have on Yammer. There's a thread where people share photos of their dogs and there was a thread about new food items at the state fair. We're currently working on a promotional campaign for Yammer and I've suggested we have an open group where people can talk about non-work related topics to encourage them to socialize with colleagues and get used to the idea of Yammer. I'm not sure how it will work, but I know my husband checks Slack for dog photos and work messages in his off hours.