I know, that my login credentials are sent in clear text for every body to read, when I log into facebook without using https from my mobile phone's browser in an public / non encrypted wifi.
And I assume, that official app encrypts my login credentials and the content transfered between their servers and my device.
But I googled and found no information if this is true. I found only posts explaining how to use HTTPS for the browser based login, third party encryption apps and how to make your own app work with facebook's encryption.
[update] Thanks for the comments so far. I understand that you need to use tools like wireshark to really ensure for yourself, that apps encrypt the data that they transfer.
But I am flabergast, that it is not widely known or documented if and how the facebook app uses https or similar means to protect its users in public wifis.