Facebook is a great place to do competitions because every time someone engages with it (by reacting, commenting, or sharing), it is spread to all of their friends. So if you have a good enough prize, you can reach lots of new people.
However, 'Share to Win' competitions are not the right thing to do. If you're running a competition, don't use 'share to win'. And if you spot a competition which is 'share to win', don't enter. Here's why.