As pointed out before, in this blog and when covering this space at Gartner, most consumer file sharing solutions (several well-known vendors) allow files to be uploaded to their platform without antivirus (AV) and malware scans! You might think that this is not an issue, because your company scans for malware at the gateway and at the desktop, so what is the risk here?
The problem is not when employees are working in your corporate environment, it is when they sync corporate data to other devices (bring your own device to work) when outside the network. Employees work from platforms which are not supported by corporate IT policies or protected by corporate security defenses; however they are commonly used to share and send files to customers, partners and other business associates.
Companies are simply not able to force employees to have antivirus at home, however home computers and other personal devices are increasingly being used for work related activity. This problem typically flies under the radar because corporate IT does not have the resources and funding to deal with it, as they are forced to focus on more visible projects and issues that are obvious to their managers. What is seen as an efficient tool by employees, is a headache from a compliance perspective. Even if there is a policy in place, workers will ignore it simply because they need these unsanctioned tools to get their job done. And that is probably why you are reading this post in the first place.
So ask yourself, how many of your employees use a Mac to work at home and use consumer file sharing services that sync to Macs? Because that might expose you directly to this problem http://www.bgr.com/2012/04/24/though-risks-are-minimal-one-in-five-mac-computers-found-to-contain-windows-malware/