Facebook just announced a new messaging system that isn't quite email or IM but something inbetween. There's plenty of coverage of the press conference out there so here's my first thought for now - this is about the display advertising market.
Facebook already serves 25% of US online display ads. Commercially, that puts it in direct competition with the two gorillas in that space - MSN and Yahoo. Both of the incumbents have a lot of advantages selling online display inventory - a mix of premium content with huge quantities of cheap network inventory, their own third-party ad network, behavioural targeting data and tech plus the best buyer relationships in the business. But those fortunes were founded on - and still rest on - the mass inventory that comes from running the world's two biggest webmail services.
The question to ask about Fmail is not about the tech or the user experience but about the money, the money, the money. FB, that is already taking over the display ad space, just announced a product that could cripple the foundations of its two main competitors. That's what Fmail is really about.
(This post was written on a phone and has subsequently had the links added.)