first, Seems that you mean for held coins on Lutpin not Luptin.
I think that not fair your offer. If you do not take 15 participants, you will cancel the contest, and you don't have any obligation to pay anyone of people who's promote you for free. So if there are only 14 users, you will get free fb shares and no one will be paid for this. Mine suggest making the offer where is always minimum one participant earns something. Like: 15 participants > winner take 0.04, 10 participants > winner take 0.02 etc..