Including social sharing buttons in your email is a great way for contacts to share it with their social media networks! With that said I can see you are still working in our older Legacy Newsletter Templates, so the steps to do this are a little different than what's shown in our Help Center. The available article for inserting social share buttons is for our Third Generation Editor. To add these buttons in your most recent email you will have to click on the header of your email.
Then select Social Share Links in the header pop-up you receive.