Despite plummeting device sales, BlackBerry (NYSE:BB) continues to hope its device business can see a turnaround. The company even announced a manufacturing agreement with Foxconn at the end of 2013 for the joint development of a consumer smartphone for Indonesia and other fast-growing markets in 2014. Still betting heavily on its device business, BlackBerry investors should hope to see signs of a meaningful consumer brand -- at least among its existing customers. But a leaked employee email revealed by TmoNews suggests there may very little loyalty among BlackBerry's current customers.

In the email, T-Mobile outlines how successful its promotional program for BlackBerry customers to upgrade to new devices was. It was so successful that it boosted BlackBerry trade-ins by 15 times the normal rate.

But as Fool contributor Daniel Sparks explains in the video below, the most surprising fact in the leaked employee email was the percentage of BlackBerry customers who opted to switch to different brands, despite an even greater incentive to upgrade to another BlackBerry device.