How safe is Yammer or Present.ly

TechCrunch50 winner Yammer or Present.ly which are microblogs just like Twitter but “only” for employees. Why would Employees use such service and how safe is Yammer or Present.ly ? I can understand to have such service internally but external data sharing is something I would never allow my company to go forward. If my info are stored on Database , that DB better be on my network and not somebody elses.
The concept of the two startups is awesome I love the idea, and I can see Yammer or Present.ly work for things like:

@LiveCrunch Let’s go to lunch

@Bob Can you help me with WebSphere?

Yammer get’s hacked, then what? Everybody knows what my employees are writing and everybody knows what software I am using.