I would suggest that Yammer explore integrating Twitter feeds into its interface too if possible. We’ve seen over the last few years that employees do not always separate their organization relationships from those outside the organization whether it be in their professional or personal lives. That’s why Facebook and LinkedIn are often used as business tools too. A single consolidated interface would go a long way. Otherwise, its like having two separate email programs – one for email to fellow employees and one for emailing people outside the organization.

Admins can permanently remove the account.

That’s exactly what would happen. Each employee should have a unique email address and given the admin interface, it shouldn’t be hard to locate a specific user’s account. Search their email, find their account and hit delete.

As for maintaining two separate accounts (Twitter and Yammer), it all depends on what you are looking to get out of each network. Sure you can set up a private Twitter account and only approved users can see your tweets, but it still puts all of that into the public domain. If you only want to have private conversations, why not do them on a private network like Yammer? Also, if you’re concerned about the process for removing a former employee from Yammer, what about the administrative process that must exist to approve users to follow your executive. It seems like that would be much harder to manage then approving/deleting user accounts on Yammer.

I would venture to say that most people using Twitter from a business perspective are doing so to interface with the outside world (ie. their customers), and not using it as a private network of internal employees. If that’s what you are looking for, I think there are better platforms out there to do that.