Political discussions with general public have always been like that, in every country and every age. It's not Facebook's problem, that's just how people are.
Any topic that indiscriminately concerns almost any person (e.g. politics, religion, weight loss and bodybuilding, how to parent and educate children, cooking, healing common pains/sicknesses), when discussed with random people, is automatically doomed to turn into a mess of uninformed opinions, misinformation, trivial logical slips and ordinary quarrel.
Tech/science discussions are usually constructive because they imply harsh entrance filter on education and intelligence, because most of those who upvote "that is not right!" can not join a public discussion of pharmacokinetics of a certain organic molecule no matter how welcome they are. When filters are absent, you get what you have described.
The real problem is that over 95% of people do not even value being constructive, checking facts, citing sources, etc. That is not a technical problem, you can't solve it by tweaking voting buttons.