That is to say they already have my email address, it's there probably memorised in their email app, and yet they use FB.
That's one scary power FB got there. Suddenly email is not open anymore. (Though I don't blame FB for it: they did business, didn't force it down anyone's throat).
It's really hard to use Facebook without it taking total control of your social interaction.
Probably lots of kids these days don't even know this, but there was once a time when your (probably only) email address was supplied by your university or employer. If two people moved, it was easy for them to lose contact with each other until a "chance" encounter (e.g. seeing them post on Usenet or whatever). Facebook solved that problem by making everyone's address book self-updating.
I actually read a fb message in my normal email box and if it's something actually valuable, ill then respond on FB.
And that's the important bit, people seem to either check FB mail directly or read it in there own email. Either way you've got more chance of getting through to them soon if you send via Facebook... I don't like it either though, it makes me rather uncofortable to have more comunication going through FB.
My experience with non-geeky friends is that they check facebook for messages and interact with friend/family through comments and wall-posts. Some of them get email notifications for these things but they use them as a prompt to go to facebook.com to view and respond. Email is the medium but not the message.
There are still other e-mail features FB private messages don't offer: forwarding message threads, attachments, email subscriptions, etc.)
Note 1: check out the embedded video Note 2: take the rest of the content with a pinch of salt; this is an email marketing vendor's blog!
For a huge population, email never worked very well, mostly since they had to find out someone else's email to talk to them. Facebook fixes this completely for any personal email use - all your real-life friends are friends on Facebook.
I'm guessing a huge percentage of personal mail has moved from email to Facebook messages - does anyone have any data about this by any chance?