We have many different 'tags' you can add to your content, like:
{JFBCLike} - This will show the Facebook Like button
{JFBCLogin} - This will show the social login buttons
If you have a site where user's can add their own content (like a forum or blog), the social render key is meant to be a 'secret' key that is required for the tags to work. That makes it so user's can't add their own JFBConnect tags and have them functional on your site. To use the key, enter a value in that setting above, and then use it in your tags, like:
{JFBCLogin key=1234}
I hope that helps explain,
Alex