There is absolutely no evidence that you can pay to get into trending.
If there were, it would be a much bigger story than this flimsy article which is rooted in the editorial judgements of a few contractors.
It really annoys me when these drive-by insinuations about "promoted content" are made. Digital media companies—including news outlets, Facebook, and Google—all actually are careful to label anything which is paid placement as such. All these baseless accusations of unlabeled paid placement do is undermine the moral standards for labeling paid placement.
If you are going to make extraordinary claims, please bring extraordinary evidence.
Companies providing perks, quid pro quo services, pre-written news stories and other non financial advantages are part of the game. None of those would trigger any of what would be labled as paid placement, but are still promoted placement.
It is also worth noting that FB trending topics can be tuned.