Someone please help with facebook?

[Derek]

I believe this has something to do with the device and version of the app they are using to message with. Some versions may offer the feature and others may not.

This time stamp must be either when their last activity was with the app, when they last sent a message, or how long their device has been idle. Most likely it is when they were last active within the app, either opened it for a second, or answered a message.

Basically gives you a general idea of how long they have been "away" or inactive. It should be fairly accurate.