I'm not sure I'd trust statistics put out by Posterous on usage given they still consider counting bot hits as genuine page views an acceptable practice.
We're looking into upgrading the public counter in a future update as well.
Now what they have created with this group feature is a way for groups to easily communicate and stay in touch.
This allows teams to stay in touch without the need to have a wiki, a private wordpress install or any other solutions which just do not work.
In my opinion, this is more powerful than Yammer and the likes for the very simple reason that Posterous Groups does not DEMAND web workers (who are already busy with work) to change their behavior to update everyone and stay updated. It all happens from everyone's favorite application, email.
So I think Posterous will gradually realize the little tool which now accounts for 20% of new sign ups is something companies will gladly pay for.
And guess what? When a new employee shows up, they already have a knowledge base to learn from.
I think Posterous is gonna end up going Enterprise.
