Apparently, there was a "glitch" last night:
Last night, in an embarrassing glitch for Facebook that raises questions about privacy on the site, some users of the social-networking service began getting hundreds of personal messages that weren’t intended for them.
A Facebook spokeswoman emailed the following response to Digits: “During our regular code push yesterday evening, a bug caused some misrouting to a small number of users for a short period of time. Our engineers diagnosed the problem moments after it began and are working to get everything back in its rightful place. While they fix the issue, affected users will not be able to access the site.” The company said it was still investigating the problem and could not immediately respond to specific questions about the glitch.
It is unclear how many Facebook users have been affected by the problem, but several Twitter users reported the glitch. A user with the handle seantanu wrote “Some bug: Facebook messages intended for others delivered to me today. 71 and counting,” and colleen02127 wondered if her problem was a “facebook fail”. Silicon Alley Insider reported on the glitch last night and said it appeared to be affecting people who joined Facebook soon after the service started.