The company where I work tried to use Yammer in 2009, and it seemed like a good idea implemented very poorly back then. It was just the engineering division with about 40 people. Tech issues quickly went to email threads where you could deal with code, screenshots etc. It mostly got used as an IRC channel to post WFH messages. But what caused their downfall for us was the constant updates (using AIR--argh) that broke the client, moved buttons around, etc. One by one, people just didn't update, then just removed it (AIR too). Since I hadn't heard much buzz about them until now, I thought that happened to everyone and they died a slow death.