If not entirely unfounded, security fears commonly associated with the public Cloud are, at best, vague
Risk represents the crux of the matter in the public cloud vs. private cloud argument. If the analyst community is to be believed, private cloud evangelists are getting their way. Not surprisingly, the private Cloud is growing—providers have positioned their services on the promise of data security. Enterprises, built on a foundation of risk mitigation, find it tempting to look no further. But while there are tangible benefits to be gained by adopting the private cloud, better security is definitely not one of them.
In fact, risks inherent to traditional IT environments are largely retained by shifting to a private cloud. On the other hand, many of these can be substantially mitigated by migrating to the public cloud.
Large public cloud providers invest large sums to secure their infrastructure from disasters and to protect the privacy interests of their clients. They deploy state-of-the-art, multi-factor access control and electronic surveillance at their data centers. They must also comply with stringent regional security mandates, such as the EU Data Protection Directive.  
Providers offer encryption using AES 256—a secure, symmetric-key (256-bit) encryption standard. Some of them, like Salesforce.com, issue a session cookie only to record encrypted authentication information for the duration of a specific user session. This does not include the username or password of the users. Instead, it implements more advanced security methods based on dynamic data and encoded session IDs. Moreover, public cloud providers are legally bound to review, share, distribute or reference client data only to the extent defined by the master subscription agreement. Effectively, it is impossible for a particular public cloud tenant to access your data.
In addition to connectivity and power redundancy, most public cloud providers offer redundant configurations for each infrastructure component. Client data is often stored on robust hardware and replicated across several data centers globally. If servers in a region are hit by hackers or a natural disaster, client traffic is simply diverted to one of the several other data centers.
Intricate access control perimeter, military grade encryption and multiple data centers are all prohibitively expensive if one were to go the private cloud way. Public cloud providers benefit from economies of scale by extending these features to their clients. In short, public cloud providers would be out of business if customer data was not safe!
If you’re at a pivotal point of decision making, a vendor-agnostic Cloud assessment framework can help you chart out a cloud adoption roadmap. Most organizations are not fully aware of which applications are well suited for the cloud and why. Your decision should be based on a comprehensive Cloud assessment framework, including a product roadmap, business case and flawless architecture—and not on some vague security threat, amplified by a private cloud reseller.