- Google or YouTube search history helps show me more relevant search results. Ditto for Chrome browser history.
- Google Android's location tracking means I can look back at my timeline and see where I've been on any day. I can see which countries and cities and restaurants I've been to. That's neat, kind of like an automatic diary.
- Facebook keeps track of my likes and interests and shows posts and ads that are more relevant to me. How is that worse than getting generic posts and ads?
The only bad effects I can think of are if Google or Facebook starts leaking all my data, and it being personally identifiable. Luckily, as a profit-seeking company, they have tremendous incentive not to do this (especially with so many privacy obsessed people keeping them honest).