Facebook has presented a function for generating »heatmaps« of users at e.g. natural disasters. Techcrunch explains:
A new initiative from Facebook will provide aid organizations with location data for users in affected areas, such as where people are marking themselves safe and from where they are fleeing. It shows the immense potential of this kind of fine-grained tracking, but inescapably resurfaces questions of just what else the company could do with the data.
Naturally, it is a good thing if Facebooks collected data can be used for saving lives.
But you should remember that this sort of technology also can be used for surveillance and that similar data can be sold for commercial purposes, without your explicit consent.
Techcrunch: Facebook will share anonymized location data with disaster relief organizations »