You probably don't realize that you've been paying for social media all this time that you thought it was free. No, you haven't been charged on a monthly basis. Instead, if you post pictures, videos, or any other content, you've been selling social media sites limited licenses to use your photos, and content, pretty much anyway they see fit. That's right, your favorite funny face profile pic could be emblazoned on an IRL (in real life) billboard next to a caption to sell the latest in fast acting laxatives.
While it is highly unlikely that any large social media site would go that far, most of their terms of service would allow them to. Generally, by agreeing to the terms of most social media sites, including photo sharing sites, users grant sites the right to use their photos for any purpose, including advertising, and even for re-licensing. This all means you might not be able to sue if you find out one of your photos got used unbeknownst to you.