We can extract early insights into statistical trends through detailed study of records kept for possibly unrelated purposes. I think of it as digging a Weak Signal out of noise.
There is a sense of surreptitiousness to big data. It might be possible to mine my mental state from my cookie trail but those who know me just ask, how you feeling today?
The internet makes it possible for corporations to prosper scraping up the table scraps of information I leave behind. Good for them. But there is no meal for me there.