So what social site were you on when it rebooted? And are you saying it rebooted when you were talking on the phone as well?
Random reboots have been known to happen on various devices, and it can be hard to tell if it's due to hardware, firmware, or some app you might have installed that's causing problems. The first thing I'd suggest is to boot into Recovery and clear the system cache partition, which can sometimes correct these kinds of issues. You won't lose any personal data.
Reset: Samsung Galaxy Note 3 | T-Mobile Support