Increasing quality of Facebook shares bounty campaign

Before establishing the bounty campaign I have been exploring many others campaigns which can find with google search on the web.
There are almost the same Facebook shares bounty task conditions.
Everybody counts friends of participant, age of account and other lesser details.
But when I started bounty campaign with such bounty task I found that these conditions are not working in effective way.
Most bounty hunters share many posts of bounty campaigns they are registered in.
And most of these posts are not liked by anyone.
And now I'm sitting here and asking myself: "Why all these bounty tasks all over the web are similar and such not effective?"
What reason to give tokens for such useless sharing? Most of such shares brings zero attention.
And how to increase quality of this bounty task?
Maybe create condition of rewarding only if last avarage X posts on the hunter's wall have X likes from different X friends?
I understand that this condition is not very strong, but this is much better then no conditions at all.