Not only is all of this tracking and hyper-targeting super creepy, it can have unintended consequences. For example, bad actors can use these individual targeted ad systems to influence or manipulate people (e.g., impact political movements) or segment groups in a discriminatory way (e.g., restrict ads for jobs or housing).
Even worse, these companies can keep all this profile and tracking data forever — and combine it with other data that they buy from data brokers (like retail purchase records, magazine subscriptions, telephone records, etc.) to make individual profiles even more comprehensive. Government agencies, police, or anyone else can then require Facebook and Google to turn over these incredibly revealing profiles of you, your friends, or your family.