I do not really understand why this would need a hide option o_0. If something needs a hide option it is the Users Online list, if I am busy and do not want anyone to bother me with Rmails at that time. Still Users Online does not have hide option.

I support.
It would help so much when I want to offer on a trade that was set up two weeks ago for example. And I do not know if the owner just forgot to delete it, or they went inactive that's why the trade's still up, and my offer will rot for a week until I realise...

I think the "Last Seen" thingy could show when that user last logged out. Not the time when they last logged in. So it wouldn't show if they are currently online or not. And the time shown would change only when the user logs out again. That way, it would be also useful if I saw someone on SB a few minutes ago and want to contact them, I could see if they logged out in the meanwhile.

Also it doesn't have to show the exact time, like  "Dec.12. 8:45 a. m." It would be enough if it just show the day, like "Dec 12.". Or if it is too much, it could just say like: "Within 24 hours" or "Within 2 days" or "More than a month ago".

I do not really see how this would hurt anyone. But adding a hide option would actually defeat the original purpose o_0.