Earlier quoted context omitted.
You could inspect the packets yourself to see that they are encrypted.
how do i know they aren't sending the chat log in a side channel, encrypted?
So I fail to see the problem. It still likely protects you from nosy neighbors or nasty non tech savvy competitors, even if it doesn't protect you from state level actors or from Facebook itself.
The above, in addition to the fact that open source is neither required for auditing, nor guarantees proper auditing occurring (see OpenSSL having vulnerabilities for years before anybody released them to the public)