WhatsApp seems to be contributing, being used for, the source of a lot of and ended up as a vehicle of fake news and misinformation.
It is being used as a carrier of circulation of forward as received information without quoting the source or reference of the that information.
Majority of people do not have the time or inclination or care to investigate as long as it fits in their belief system and simply circulate to other groups. If it does not fit the or belief system they simply ignore it.. As a result misinformation stays as misinformation, does not add any value but instead confirms their cacooned belief system.
As the the WhatsApp groups are small and people know each other no one dares to question for fear of others being annoyed.
Whereas in case of Facebook your posts are open to a large audience and you stand a chance to be listened to, to listen to counter points of others and course correction, provided you are favourably disposed to listen to criticism, open minded and are ready to leave your topic of discussion open ended for others to take the lead.