First, click the 'Share' button located in the Editor toolbar, as seen below. (You can also use the 'Share' option found in the thumbnail preview on your Dashboard, but this will not generate the Open On Web URL that is generated using Share from the Editor).
Next, ensure there is public access to your URL, as shown below. Choose 'Anyone can view the sharing link'.
Finally, choose the Social Media icon where you want to share your work and follow the instructions in the popup that appears.
Note that if you add a downloaded PNG version of your visual to Facebook, it will shrink automatically to fit dimensions set by Facebook. (Alternatively, for the PNG upload to Facebook, you can include the Open On Web URL in the body of your Facebook post to allow your audience to view the visual live in its original form, including hyperlinks and videos you may have added).
Facebook, Twitter, Google+ and LinkedIn icons are located in the general icon collection in the editor. Use the search function to locate the one you want.
Additional ideas and tips for sharing Piktochart visuals on Social Media can be found in this blog article. It sheds some light on best practices, and is full of practical advice, including an up-to-date listing of social media image dimensions (so that your Piktochart creations are always the right size). Get creative!