I have a theory. Any input is appreciated. I understand the facebook chat mobile icon has to do with the last time a person has logged into facebook from their mobile phone. It does not time stamp any activity from a desktop. 23m means they logged out 23 minutes ago. 3h means 3 hours. 1d means 1 day ago. Then sometimes there is just a mobile icon. I think that it means they logged in from a regular computer and not the app so it shows they have it installed on their phone but were last active on a computer. Or they turned their phone off. One of the two. I am leaning towards logging on from a computer which resets the time stamp.