There's not much to say about it. I've read some other posts from really reputable blogs and for whatever reason they're all padded with nonsense. Facebook Mail is an extension of their messaging system that incorporates email, has a kind of priority inbox, and is all based on people rather than threads based on subject. Facebook is aggregating all the ways you communicate to better simulate a conversation as though you were standing next to the person.

It's a very specific style of communication that speaks directly to teenagers and the way they communicate. While teenagers could and should use a communication method that caters to them, and the new Facebook Messages does that really well, it's not for grown ups, or business, or anyone with any depth or complexity. It's advanced IM(instant messaging).

Grown ups need threads by subject - whether they choose to organize their inbox as individual messages or threaded, the subjects still differentiate the content of an email or conversation. I may do a lot of work for a client, the absolute last thing I would ever want is to scroll down a bit too far in the conversation and see specs for an old project and incorporate it accidentally in the new one. Or for the client to pitch an idea for a new project, which get's muddled with the current project, or requires a paragraph explaining that it's a different idea and just a pitch and has nothing to do with the current project. One of many examples where subject based organization is a far simpler paradigm.

Of course we all have a adolescent side - some more than others, and it's that same aspect of ourselves that Facebook appeals to when see what old friends are doing with their lives. It's that same part of our brains, that couples with short sightedness, that makes some people think that peer pressure is more important than preventing access to your personal online life from random third party applications. Why else would anyone do those stupid Facebook Quizzes? It explains why some people believe there's enough value in completing a quiz to give up the keys to their personal life to some shady app developer. Or at least the same short sightedness that becomes so overwhelmed by the social pressures of Facebook that they've never read the TOS or Privacy Policy. Never mind Facebook's but of the third party quizzing application who can do whatever they want with your data.

The two other issues with integrating Facebook Messages with your email is lack of respect and trust. Giving someone your Facebook email address is an immediate indicator that you don't want communication with that person (or company) to be too complex. You're saying, "Here's my Facebook Email, I'd prefer it if everything we say to each other from now on was part of the same conversation cause DUuuueah I lack the ability for context". As for trust there are two main categories of people. Those who trust Facebook (unfamiliar with their history, never read the Terms, unaware of the frequent privacy control resets, etc.) and those who don't trust Facebook (people with common sense, people who don't live in a bubble, people who can read).

While I've recently had to reactivate my Facebook account after two years so I could access their developer API for work (I got out during a significant Intelectual Property and Privacy blowup in 2008), I refuse to put any information on it that isn't already public. Why, because I don't trust them. Facebook has done a lot of things in their past that are notably untrustworthy. Their whole approach is based on peer pressure and pushing people around how they see fit, their privacy controls are only meaningful until they decide they're not - which happens frequently enough, and they don't appropriately abstract apps or themselves from your personal data.

Why would I trust a free service like Google with email and documents and stuff, but not Facebook? Well for one I know that Google has serious restrictions dictating which of their employees have access to my data, even those few select people are meticulously logged and audited, and any violations result in firing and tighter security. On Facebook I have no idea which of their employees can access my data, and I know that at their whim a mere Privacy Policy update could result in all my controls being reset and my private information entering the public domain forever. If the controls are meaningless, and access is ambiguous then there's no trust.

For a simple analogy; Google is run as a republic. Microsoft is run as a corporation, Apple is run as a dictatorship, Facebook is run by a young dictator.