If you want to share your magazine on social media, go to "My magazines" --> "View Magazine" at the desired magazine.
When you want to share your whole magazine on social media, click on "Share" at the bottom left. If you want to share the cover, a specific page or a cut-off, click on the "arrow" at the top right corner.
If you don't see any buttons, make sure that "Social sharing on Yumpu site?" is activated in the magazine settings,