What are the copyright issues with sharing music and videos that I find on social media? Not ones that I upload, but ones that I find besides my own. Like when I hit "share" on Facebook. Have I violated copyright law by doing so?
The legality of "sharing" ultimately depends on technical details of the platform, but the simplest answer is that sharing a link is not copyright infringement. If all you provide is a link to a file and the platform just distributes that URL (does not copy the content), this is not copyright infringement. This is true even if the file "out there" is illegally posted. You might be liable for contributory infringement if you're knowingly encouraging illegal copying, by massively distributing url's to illegal material.
You might be in violation of the target or source platform's TOS if you share a link to material contrary to their terms; and violation of the TOS means that you have infringed copyright (copyright law is the teeth behind TOS).
It is not illegal to give a link to an artistic, dramatic or any other type of work that's copyrighted on social media. Do they have content under a licence is your question. Most websites and organisations, such as Wikipedia, will definitely have a Creative Commons license for any material uploaded.
You can share a link to a photo or video.
There will be an EU Directive that will (may) forbid linking to news article and things. EU Directive for the Digital Market I think it is. Currently, many are opposing towards it.
Asa others have said, when "sharing" means providing a link to existing content, that is not copyright infringement. However, when "sharing" involves actually making a copy of the content and distributing the copy, that could be infringement, depending on the specific facts.
Also, making a "deep link" that bypasses an intro or login page when the owner has clearly said to link only to the surface page, may be infringement. There have been cases that held this to be unlawful, as it might deprive the owner of revenue gained by requiring a login, or distort he owners expression by depriving it of context. This again will be a very fact-dependent situation.