It's like wikipedia. People read things and think they are true, when in fact they haven't been vetted at all. Somehow it has gotten a reputation as a good source and it's been a big problem. I don't think facebook has that same reputation, so it's not as bad of a problem.
Freedom of speech, people can post what they want but that includes false news and it can at times be difficult to discern between false and real posts. On the other hand, it can teach people to check sources and conduct some research to see if the posted news story is real or not.
It really depends on what your definition of news is. Facebook definitely has a lot of false news, but so do cable networks. Cable networks spin the news in anyway that benefits them. It's very hard to find unbiased news today. I don't think Facebook is the only culprit of this.
There is so much false news on Facebook that it would be ignorant not to double check facts. But Facebook is not the source of false news because on Facebook people are simply posting links of articles containing this false news. The internet is so amazing because it enables its users to have access to unlimited news/knowledge, but also makes it extremely difficult to discern what is to be believed.