Facebook's Mail Will Not Be A Gmail Killer
I doubt it.
True. The bulk of Gmail users, in my experience, are there not just for e-mail. They are there for the integration of Google's other products like calendar, text and video chat, phone calls, SMS, documents, etc. Facebook has some of these features, but they're not used the same way. For example, people who use Gmail for work and associate their inbox with their various calendars do so to help organize their business lives. Facebook's calendar service is for social engagements.
The only other area where Facebook's and Google's services overlap is chat. But Facebook, at present, is harder to establish a chat relationship. Chatting withing Gmail requires an e-mail address and approval - but it doesn't open both users up to seeing all other personal relationships such as friends, likes, etc. Therefore, Gmail users are more likely to establish a chat relationship with business contacts than Facebook users.
For everything else, Google has Facebook beat. I don't see any sensible reason for Facebook to offer an apps engine that will compete with Microsoft. However, I do see Facebook offering an SMS and phone calling feature. It already integrates with Skype for phone calls. But these are still mostly social relationships - friends, relatives, community acquaintances. These are not serious business contacts.
Granted, with 500 million users already signed up, Facebook has an advantage in launching a fully functioning mail system. But I don't see the "Fmail" system as being the juggernaut that so many tech blogs have predicted, unless there's full integration with other services. And given Google's recent move to sever ties with Facebook, Gmail integration with Facebook's mail system isn't likely.
So, that will still keep Fmail behind Facebook's walled garden, and the people most likely to use it are the types who find Hotmail a satisfying experience, teenagers who are just establishing an online presences, and moms who check e-mail once a week.
That doesn't mean no one will use it. I will probably use it, and will probably find myself using it to send messages outside of Facebook from time to time. But it will not be the primary e-mail address for the millions of Facebook users.