One of the more popular cloud based applications, in fact one that preceded the popularity of the cloud itself, is online backup. Use of services like Carbonite, IDrive, and Mozy have been widely used for years.
But are they safe? We all know about the successes of hackers in getting inside sophisticated and well protected systems. Why couldn't they hack into these services?
It's probably unwise to give a definitive answer in these days of ever more sophisticated hackers. However, there is every reason to suspect that such backups are very safe. The reason is the encryption systems they all use, which are among the best, and give you control over who can decrypt your data.
Time and again, encryption has been proven to be one of the most valuable security tools. Data is no good to hackers if they cannot read it. If they do manage to break into the cloud backup providers system, they also need to break into your computer and steal your encryption keys. A difficult job.
It's much easier for them to break into your computer and steal your unencrypted data, because most people do not encrypt their hard drives. So arguably, your data is safer being backed up in the cloud.
Here's another take on the issue.