Some things to consider
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Twitter's Terms of Service stipulates "What’s yours is yours — you own your Content" but if you posted something that isn't actually yours, the person who has the copyright can report a copyright infringement.
Even though you can report copyright infringement, It is rare that a Tweet would be covered under copyright because of the degree of originality and medium tangibility required under the law. However, if your Twitter account leads to a book deal or tv show that would result in copyright protection of the original work.
By signing the agreement, you give Twitter non-exclusive rights. This means they are allowed to use your work to "promote and improve [Twitter] and to make Content submitted to or through the Services available to other companies, organizations or individuals for the syndication, broadcast, distribution, promotion or publication of such Content on other media and services".
So Twitter can use your work but other's can't.
The whole premise of Pinterest pin (post) someone else's photos. Even though there is usually a link to the original post, you probably break copyright because you did not explicitly obtain the copyright holder's permission before pinning. Site users are responsible for the all content posted so if you are sued you must pay to defend yourself and the cost of Pinterest's lawyers. For companies who don't allow you to pin their photos, there is an opt-out code to add to your website.
When an article is published, the author usually has to sign a publishing agreement. Each agreement is different but usually has a stipulation that the author can't make the work open access for a set amount of time. When uploading your work to ResearchGate you may be in violation of the agreement you signed. Many researchers knowingly upload their work regardless of the copyright but publishers have taken ResearchGate to court for facilitating the sharing of restricted material (estimated at 7 million articles).
Before uploading - check your license agreement. If you are unsure, the NU Digital Center can help.
Youtube has extensive documentation to help you understand copyright. These links explain what music and video are allowed on their site as well as what to do is someone steals your works.
When posting a video, it is not enough to post a notice in the description about not having rights to the content. If by posting something you are taking away views from someone else - this violates the copyright.
YouTube tries to prevent copyright infringement by cross checking every video uploaded against the other uploaded videos (similar to Turnitin)