It's going to be pretty high standards of proof to give this anything that resembles credibility.
What sane person would ever trust Facebook to keep something private? If anything, the addition of "secret conversations" will be more intrusive, because this service delivers more nodes to the social graph: how many secret conversations, with whom, where, at what times, etc. Facebook and the global spy regimes it provides content for care more about building a network of associations than actual content. Who cares about the needle when you can control the haystack?
Edit: and it is http://www.whispersystems.org/blog/facebook-messenger
For me personally, they've lost a huge chunk of credibility by lending credibility to companies for apparently rewriting the definition of "end-to-end-encryption" from previously referring to the users as the end to now referring to some magical proprietary blackbox as the end.
And the other chunk of credibility sort of died off, as they apparently seem to think depending on proprietary Google software for their products is how you do privacy.
That is, their Android-client for Signal depends on Google Play Services for receiving notifications, even though LibreSignal exists, which is a fork specifically to remove that dependency, and from which they could have easily pulled that changed code in as a fallback.
And when they figured they should make a desktop client, apparently the best technology that they could think of, was a Google Chrome extension.