Saw this interesting map recently, which defines areas where people call each other the most.
From the most excellent Strange Maps Blog:
From the article:
"The extension of these areas was calculated by MIT and IBM, analysing anonymised call data. The map delineates zones in which people are more likely to call someone inside those areas rather than outside of them. The result is a revelatory re-mixing of states of America. Some split, others merge with their neighbours. "