I find this promoting option available as a good idea in general.
On the other hand, if there is only one criterion (and one place) where those top trending posts are displayed it's clear upfront that only those with thick wallets would be able to participate and finally it might all end up that only those with already gained higher reputation would compete among themselves. - That's not good!

I.e. on Facebook (although I'm not the big fun of it) even those with the small budget can gain needed exposure through various targeting ways.
Therefore, I think this initially good idea should be more thought through and developed in more details.