Stefan Wuchty and Brian Uzzi at Northwestern have published a rigorous paper suggesting that besides volume and reciprocal exchange, response time can help identify different types of ties in one’s social networks.
The Wall Street Journal coverage is here: E-Mail Separates Friends from Acquaintances.
The Science Now coverage is here: E-mail Reveals Your Closest Friends.
The original research paper is here: Human Communication Dynamics in Digital Footsteps: A Study of the Agreement between Self-Reported Ties and Email Networks.