Confirmed by numerous people, including very close friends of mine:
"Dear Facebookers - Your private inbox messages are now visible for all to see from 2012 and earlier. This could get VERY awkward, very quickly...this is a new change from Facebook as of yesterday. Don't believe me? Read your timeline from those years...yikes! They look like wall posts.....they are not! There's an easy fix--when you're in your own timeline, just click on the year on the right (2007, 2008, 2009) and you'll see a box with a bunch of friends' messages and a header that says 89 people have written on so and sos timeline--hover over the right until you see the pencil, click and select hide from timeline. Repeat for each year. I went through and hid all of mine. ALL messages I had been sent were visible, though my replies were not. "
Also: One of many sources
(Check the comments).
"Dear Facebookers - Your private inbox messages are now visible for all to see from 2012 and earlier. This could get VERY awkward, very quickly...this is a new change from Facebook as of yesterday. Don't believe me? Read your timeline from those years...yikes! They look like wall posts.....they are not! There's an easy fix--when you're in your own timeline, just click on the year on the right (2007, 2008, 2009) and you'll see a box with a bunch of friends' messages and a header that says 89 people have written on so and sos timeline--hover over the right until you see the pencil, click and select hide from timeline. Repeat for each year. I went through and hid all of mine. ALL messages I had been sent were visible, though my replies were not. "
Also: One of many sources
(Check the comments).