We implemented Yammer some years ago to improve internal communication and transparency. People were at the time complaining that they didn't know what was going on in different departments and locations, and a lot of knowledge was lost in email silos. We decided therefore to explore the value of enterprise social networking.
If you don't know Yammer, then think of this as Facebook for the enterprise. It has similar features and functions, but also have the ability to set up groups. You can like post, comment on things, upload and discuss files, and it has a wiki for real-time editing and discussions by staff.
People like progress, but not change. Staff that use Facebook for personal networking quickly got the value of enterprise social networking, while others felt it was just another communication channel. We tried to address this by making social networking part of business processes, but we still had to force a few laggards to post updates once a week. Yammer has now become our most important tool for internal collaboration. Email is still important for 1-on-1 external and internal communication, but group discussions happen in Yammer. I now get less internal emails thanks to enterprise social networking.
Yammer was acquired by Microsoft in 2012, and they are planning to improve the functionality even more. SharePoint will become the document repository, and you will be able to move email discussions from Office365 to Yammer. CMSWire just published an article about Yammer explaining top challenges and the road ahead.
If you need alternatives to Yammer, then check out solutions like Jive Software and IBM Connections for enterprise social networking. Salesforce has also Chatter for discussing opportunities and customers. You should also check out this blog post by John Mancini about the future of collaboration.