Besides asking the participants of your promotion to answer a certain question, participate with a video or photo, etc, you may suggest them to they follow your Facebook page or your social profiles on Twitter or Instagram. Nevertheless, bear in mind that, according to Facebook regulation, you can't request them to become fans.
Can I suggest different fan pages?
Yes, you may suggest different fan pages not also to the participants but also to the voters in a promotion. This way, the suggestion will pop up when they click on "Participate" or "Vote".
Can I ask them to click on 'Like', comment or share the contest or their participation?
a) A votes ranking.
b) A users ranking, activating the referrals ranking.
c) A score-based ranking.