For the individual, understanding how things work is important. A purchase, online or in person, is a tacit agreement allowing the retailer and credit card company to collect and save data about the transaction and match it with other data they have about you.
Facebook, Twitter, Pinterest, etc. do not consider you their customer. You are their product. Their customers are advertisers. Everything you post, comment, like, tweet, or pin becomes the property of the site. It can or will be used to serve the advertisers. Each app you install might track you in some way as well.