Trending happens regardless of political view. If they're manually suppressing views which don't align with FB management then users should be made aware of this.
I can't really seem to recall facebook making any promise about being a neutral party.
Additionally, the Facebook feed is not a linear display of content. The feed has been manipulated for a while now. So users have known for a long time that Facebook changes what they view according to Facebook's whims.
I don't know enough about this particular issue to judge Facebook but I think it is pretty well established that preventing fraudulent communications is a legitimate role for government.