having humans tagging certain stories as "not news" is not evil agenda based bias, it is spam filtering.
just because a group of people start shouting about something loudly, and the shouting gets popular, doesnt mean facebook should choose to amplify it.
for example, think of mean girls. when gretchen is trying to make fetch happen, and regina says "Gretchen, stop trying to make fetch happen! It's not going to happen!" A group of likeminded people, say republicans in this example, could be trying to make someone appear popular by shouting their name a lot, ad nauseum. that doesnt make it newsworthy outside the echo chamber.
if the republican network is more coordinated and better at discussing one topic at at time than the democrats, they can push a story higher on the trending list. people being able to manipulate the trending list through coordination isnt ideal. if you want a gamified, competitive trending list, visit twitter. letting pure popularity and activity overrule common sense is foolhearted. i have never seen facebook claim their trending list behaved like twitter (which is where it seems you are getting your ideal trending behavior concept from.)
using humans to balance the trending stories, instead of letting one ideology take it over, is a good thing. but balance doesnt always mean fair or equal. myths dont deserve equal space to debunking. intelligent design doesnt automatically deserve equal placement to more accredited theories. if something is factually wrong, facebook can remove a story from trending to prevent its spread.
plus, algorithms are going to display some kind of bias. any sort of spam filter will disfavor certain noise. is blocking viagra ads bias? algorithms written by people will lean in SOME direction. believing in unbias is naive.