Agreed, Microsoft didn't over pay for the software, they over paid for the user-base. The user base is what you want.
$8.5B influential? Honestly, I don't see what $8.5B gets you that investing $500M in a Skype partnership wouldn't have.
I'd like to think they have some grand plan, but honestly I don't think so.
I think they're just sitting on a stack of cash and getting pressure from investors to spend the cash and grow.