Complex algorithms certainly do exhibit bias, and not only that - it's intentional. The whole point of a "trending" topic is to discriminate (i.e. inject bias) trendy topics from less trendy topics.
So you're talking about only adding the bias you want it to add - but I hope you can imagine that's an impossible cause to get 100% correct. There's no rigorous definition of trending; nor is there a rigorous definition of the topics to be selected from, nor are lots of other matters here rigorously defined. There's no way you're going to do better than some fuzzy algorithms trained on real data. Critically, that tends to mean real data generated by... biased humans.
Having humans involved will distort that process, thus meaning true trending does not exist. Well, on Facebook at least.
Facebook's employees repulse me.
What make it 'horrible' in your view? An implicit suggestion that 'trending' is mathematically deduced a la Reddit's 'Hot'?
If you really care, it uses a GBDT and the newly released Facebook Flow.