Comment by logophobia

Because it means that a business (as in, facebook) knows too much about you. It's extremely invasive privacy-wise. Things that could happen:

* Micro-targeting for political advertisements (pretty bad for democracy)

* Dynamic pricing based on demographics (you can afford it, so you pay more)

* Insurance knowing too much about you (rejections based on your health, ensuring parts of the population won't be able to get good insurance)

* And just the fact that too much information being public can be harmful (blackmail, scams, etc)

* etc..