Although copyright is automatically given to the creator of content, this can change somewhat when what is created is uploaded/shared/made online. Wherever you share something new (like a photo) that site will have it's own unique set of terms and conditions. Some of these terms and conditions seem to have little affect on users, but it is important to note that when agreeing to these terms, we may be giving away our copyright ownership for certain things - or everything - we share on that site.
For example, sites like YouTube and Twitter have the right to use work you upload without infringing copyright.
If we want to keep total copyright of our content, it is important to read through the terms and conditions of the media platforms we wish to upload to, and choose carefully what we do share on these sites.
Additionally, as users of social media we might infringe the copyright of others by using something (an image, audio, etc) that does not belong to us. Wikipedia has a guide for its users to follow before uploading images that has some simple rules to follow. If the content you are uploading is your own, belongs in the public domain, or is freely licensed, you will most likely not be breaking copyright law.