Back in April, WhatsApp allowed encrypted chats, but Facebooks' own Messenging service hasn't allowed encryption - until now. WhatsApp has enabled this by default, but Facebook Messenger will still require you to go into your settings and choose the "Secret Conversations" option. You really should turn it on for every conversation as encryption is never a bad thing.
Facebook Messenger’s new layer of encryption has to be enabled manually for every conversation, rather than being switched on by default, as it is in the Signal app or in WhatsApp. That decision may be in part a compromise designed to help Facebook avoid legal and political difficulties; WhatsApp’s default encryption, for instance, has already put its parent company in an uncomfortable spot at least once, when Brazilian authorities arrested a Facebook executive in the country for failing to help police decrypt WhatsApp messages sent by criminal suspects in a drug trafficking case.