adaliabooks: Speculation for the first part, the part you bolded is a fact. Galaxy has desktop notifications which pop up when you receive a message or notification so unless you're literally AFK you should see a message or reply coming through (and hence be considered to be online)
HypersomniacLive: People playing games also show as online, so if you play a game in full screen, do these desktop notifications still pop up in a manner you get to see them? And can they be turned off?
I don't remember seeing them when I'm playing games, let me check the Galaxy options to see.
But people playing games also show what game they're playing, so you should know they are not 'available' online.
You can turn them off completely I'm sure, but I can't remember what other customisation options are available.
Edit: Ok, Galaxy finally loaded up.
Options for notifications are:
If Overlay is off (or presumably doesn't work for the game you are playing) you don't get notifications.
You can choose (individually) whether to get notifications at all, desktop only, overlay only or both for:
Messages received
Friend invites
Game invites
Friends coming online
Friends starting a game
Installing and updating status
Wishlist alerts and sales
Forum replies
Unread notifications
I believe regardless of what option you choose things like messages and forum replies will always give you a normal notification similar to the ones we get on the site (but without the pop up in the corner of the screen)