However, you are right in that any business which places or outsources core business processes onto web services without having signed a service level agreement (i.e. a service guarantee) with the service operator or having a failsafe or backup in place, is playing with fire.

Furthermore, in regard to the unilateral removal of content by the service operators, there were some great negative examples of this approach during the protests in Iran: After the big media (e.g. CNN) started broadcasting protest videos copied off of Youtube, suddenly the original protest videos were removed because the automated copyright infringement detection mechanisms considered these original videos to be illegal copies of CNN material! In other words, the users were punished for something they didn't do.