I live in the US but have friends I see in China, some of whom have one-character given names (that is "first" name in English usage, "second" name in Chinese usage). I know that often people avoid using a one-character given name by itself since it lacks weight, as said by monalisa at How should a professor address a woman student?. So where a close friend might call 曾國藩 just 國藩, an equally close friend would call 张扬 by the full name.
But it seems that sometimes in conversations and in e-mails, Chinese people do call a friend by a one-character given name. Am I mistaken to believe that? Or is it just because the people I know have mostly worked and studied some in the west and they often use English in their work? Or is it something that only very close friends would do?