I don't know what Crashlytics enterprise pricing structure is like, but if it's anything like other similar services they'll probably be on twelve month agreements with their clients, which would be complex to untie. I doubt the service will go anywhere for paying clients for a little while. If you're relying on a free third-party service in your apps you always have a recovery plan should something get bought, retired, or just plain go wrong.
They acquired TweetDeck and changed it a little but the team and registered company still resides in England.
and as you said Posterous is the other example, although I wouldn't say that's really a Twitter product. They're just letting people leave that at their own will and eventually they'll just turn it off.