It's quite easy to create the HTML code for sharing, and include it on the last page:
Facebook Share Button
But: You should keep in mind that this may be a violation of privacy laws. Because, if you include the "share" button, data (the user's IP and referer) is already transfered to Facebook. That means that Facebook knows that this person has doen the survey, even if they do not click the button! You may have to add additional information in your questionnaire to comply to the law.
There are workarounds that require two clicks on the Facebook button avoiding the above problem. Yet, you'll have to do some research on this to do it right.
We have a introduction on how to record button clicks, but it has not been translated to English yet: Klick zu externer Website aufzeichnen.