I fail to see why people want to delete their accounts when they decide not to use a service any more. Why not simply stop logging in?
With Yammer it at least makes a little sense because there's possibly sensitive business data on there.
But I built and ran a moderately successful Facebook app and people bugged us about it constantly until we built the feature, but the motivations behind wanting to delete an account just baffled (and continues to baffle) me.