As suggested at http://www.webupd8.org/2011/07/fbuntu-facebook-notifier-with-ubuntu.html#comment-258530057
Likely to be the Facebook logo for notifications, but perhaps for inbox messages show the profile picture of the person sending the message.
This functionality is available now in the graphNotificationChecker and the unifiedInboxChecker. Unfortunately the latter is only currently only available to Facebook developer accounts. :(
Bug watches keep track of this bug in other bug trackers.
Likely to be the Facebook logo for notifications, but perhaps for inbox messages show the profile picture of the person sending the message.