When you are sharing your campaign to Social media (Facebook, Google+, etc), you can customize the social share message displayed to make your post more engaging to your viewers.
For landing pages, locate the share settings under the settings menu on the top right.
Add your headline, description, and image
Save your changes, and publish your campaign. Below is what you get when you share your campaign URL after making the changes.
You can also do this for contests in the advanced settings of the campaign. Scroll and check "Customize SEO & Share Messages Settings" on the bottom of contest settings, you will then be able to update the settings.