Office 365 services, including Exchange Online and SharePoint Online, provide highly available service through redundancy of databases, replicated both locally within a Microsoft data center and between different data center locations. These mechanisms make it highly unlikely that data will ever be lost for any of these services.
Data backup is different from replication. For instance, in the event that a corrupt file is saved, that corrupt file will be faithfully replicated to all instances, replacing the previous valid copy of the file.  To be fair, both SharePoint and Exchange Online services include provisions for retrieving an older version of the corrupt file in this example. This does not get around the fact, though, that data is not backed up by Microsoft.
Third party services are widely available for backup of Office 365 services. CTP does recommend a third party backup, both for the case of retrieving a file version that cannot be retrieved from the Office 365 system, and also from the good practice of maintaining a full copy of your company’s data in more than one location – Microsoft, and the data center of the third party service.