Copyright law grants authors and artists the exclusive right to 1) make and sell copies of their works, 2) create derivative works (works based upon their original work), and 3) display their works publicly. These rights are automatic. You get them as soon as you create your work, whether it is a painting that took months to create or a drawing on a napkin. You do not have to register your work with the U.S. Copyright Office to gain these rights and by international treaty, these rights extend to most countries around the world.
These rights are considered a property right (hence the term intellectual property), and just like your house, you can sell it or transfer part of it for a limited time, like renting it to someone. For example, when a fine artist shows his or her work in a gallery, the artist gives the gallery the right to copy the work for promotional materials that promote the show, to distribute those copies, and of course to display the work for a limited time. The gallery has the right to sell the physical work, but not the work itself. For example, the gallery cannot make a copy of it and sell it to Target to put on t-shirts. That right is retained by the artist, however, if the artist wanted, he or she could transfer that right to the gallery, as well. The details as to which rights are transferred are laid out in the contract between the artist and the gallery.
The same thing happens when you upload your work to social media. Every social media site has a Terms of Service (TOS) that every user agrees to when they join, and the TOS details what the site can do with the uploaded work.
The breadth of rights transferred when agreeing to the TOS can vary substantially in each online platform.