Frank Pasquale on the all-powerful algorithm:

Far more worrying is the shady world of thousands of largely unregulated data brokers who create profiles of people, profiles built without people's knowledge, consent, and often without the right to review or correct. One casual slur against you could enter into a random database without your knowledge - and then go on to populate hundreds of other digital dossiers purporting to report on your health status, finances, competence, or criminal record.

More to reinforce your paranoia:

An inference like this may not be worth much on its own. But once people are so identified, it could easily be combined and recombined with other lists - say, of plus-sized shoppers, or frequent buyers of fast food - that solidify the inference. A new algorithm from Facebook instantly classifies individuals in photographs based on body type or posture. The holy grail of algorithmic reputation is the most complete possible database of each individual, unifying credit, telecom, location, retail and dozens of other data streams into a digital doppelganger.

If the last few years have taught us anything, it's that digital paranoia is usually justified.