One problem is that once information is collected about you, there's no guarantee use of it will be limited to the people or entities who collected it. There's

One problem is that once information is collected about you, there's no guarantee use of it will be limited to the people or entities who collected it. There's also no guarantee that the information won't be abused. Corporations get hacked in to and have data stolen from them all the time (the frequent news stories we hear about data breaches are probably just a drop in the bucket compared to those that happen and go unreported). Unscrupulous employees of these corporations could use the data the corporation collects. This information could also be sold (either legitimately or illegitimately) to others. It could be for stalking, harassment, identity theft, finding out your political sympathies, sexual orientation, etc. The data collected on you could be used by prospective employers, insurance agencies, or love interests to weed you out based on the impression (accurate or not) that they get of you or your social media "friends". Once the data is collected, it could stick around virtually forever. And good luck correcting any mistakes in it.