Vadim Lavrusi; journalism program manager at Facebook published this news in his Facebook timeline. Facebook changes it message box layout into side by side layout. Facebook is changing its features randomly. This changes in message box also one of them. Vadim said about this,
|Facebook Message box|
"The new side-by-side layout lets you click your most recent message on the left to see the whole conversation on the right. You can also bring conversations to life with multiple photos and emoticons. Now you can search by a sender’s name or keyword from the main messages view. And for easier navigation, we’ve added keyboard commands. To see the full list of available shortcuts, type Alt Q on a PC or Control Q on a Mac."
So Facebook here trying to give a new look to message box. Something different from Email inbox. Some days ago Facebook change every users email address into @Facebook.com and now this change in inbox..So it's clear Facebook is trying to catch the place of email(Gmail,yahoo mail,Hotmail etc) or they are trying to be like other mail services.
It's very possible for Facebook. They have huge number of users(more than 900 millions). If they can use these huge number of users it will not be very hard for them to become a giant in Email service.