Every time you "Like and Share" a competition, you are actually working for free.
Facebook is a place of great connectivity and reachability for advertising. Everything you like (unless you specify) will be shown to others.
A company that wants to gain reachability, has to pay a fee for ads on Facebook. For approximately 10 US dollars, you can gain 10 likes according to the ad system. In fact you can buy a like from a "fake account" being administered in India, and people who get paid to sit and "Like and Share" all day. And here you are, doing it for free.
According to an older study, the average person on Facebook has 160 friends. Thus, a single like exposes the company holding the competition to 160 individuals. And if those 160 individuals were to further like. Well, the reachability of the ad would become 25 600. Saving the company 2560 US dollars in Likes.
And what do you get in return for providing then your network? A minimum chance of winning something that probably costs less than 10 dollars to buy.
So what is a fair competition?
Many companies have a reward system. If you like their page, or if you register at their homepage and compete. Whether or not you win, you are offered a discount at their store, facility, service etc. Or perhaps even points which later can be used as discounts. Now that is a fair trade for your time and network.
So the next time you Like and Share, ask yourself.