We use Yammer. It's definitely a cultural shift. Some teams see it as simply another inbox to check, but other teams are actually really good to moving non time sensitive, collaborative discussions and announcements over to Yammer instead of email. I work for a global company so the shift to Yammer certain types of discussions has increased collaboration from a much larger geographically dispersed audience so there's actually some interesting perspectives and conversations that get started that wouldn't have happened inside of email.