The challenge with 'think I've fallen' is that you constantly have to ping the accelerometer. From a battery-life perspective, that kills the smartphone. Also, its hard to distinguish those pings from

For that particular application, its better to use custom body-worn hardware (e.g. fitbit).

Regarding spotting trends in face-to-face networks, we've done similar analysis political opinions during the 2008 election campaign.

Results here: http://web.media.mit.edu/~anmol/political-2.pdf