Weird idea, but why not have two cut offs? You could have a total weekly pool of X Monero for the top 3, and Y Monero for the top 10/20/30 out of an overall pool of 1 or 0.8 (0.6 would be too low to split IMO)? It would still be based off of the same pool of rewards, but it would essentially create a worthwhile competition for the frequent high quality posters and then a nice rewards system for the ones who use it somewhat infrequently. That would encourage the infrequent participants to participate more frequently because they get something like a reimbursement plus maybe a little extra.