An old saying states that if the product or service being offered is free, then what is being sold is you. That saying is just as true if not more true with the Internet. Social media sites gather as much information about you as possible and sell it to marketing firms. The marketing firms target advertising to people who are likely to buy their product.
Some argue that that's not necessarily bad, and if you willing to go along with that plan that's fine, but it least you should know about the plan.