We don't know IF Facebook is under sampling conservative publications. It depends on the goal of the algorithm. If the goal is accuracy, as a group recent conservative publications have a sorted history when it comes to truth and accuracy. Conservative news outlets also have a weird bias against science and like to push conspiracy theories with no basis behind them. Naturally if your algorithm is focused on how accurate the news is as a condition for landing in someone's news feed, conservative publications would be at a disadvantage compared to other news sources. It could also be that fringe conservatism is not popular with the majority of society.