The first question you should ask in evaluating any encryption or privacy-based software is, 'do I trust the provider?'
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?
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?