It came flying out of the gates, was always more like Facebook than Twitter and is targeted at pretty much all companies (I've worked at a 6 person company that loves it, a company that went from 10 to 100 employees that loves it and a company with 10s of 1000s of employees that loves it).
You've been at a lot of companies in a short amount of time. My experience has been that Yammer is not useful, for when the company was small and even less useful for a larger company. What exactly do you find useful that isn't covered by existing products?
