No way! We don't recommend you to directly incentivize folks to share on Facebook for an entry. The reasons why we haven't included a 'share on Facebook' entry option integrated into the Rafflecopter widget is because that would be in violation of promotion rules set in place by Facebook.
Here's a great resource regarding Facebook promotion guidelines and how they pertain to Rafflecopter:
We have a 'refer a friend' feature that allows your entrants to gain points by sharing a unique URL that tracks their referrals. That's a feature that's available for our premium subscription that doesn't violate FB's terms.