Is there a way to add a facebook share button on the form?

I want user who fill out the app to have the ability to share the form with their friends on facebook

Yes, you can try the Social Share widget: https://widgets.jotform.com/widget/social_share 

If you do not know how to add a widget to your form, here is how: https://www.jotform.com/help/252-How-to-Add-a-Widget-to-your-Form 

Open a new thread if you need anything else, we will be glad to assist you.