My simple excuse: when I go to work I usually log on in the mornings and leave the window minimized, checking throughout the day. Sometimes, if it shows you've been logged on for 4 hours and haven't posted anything, you'll get a couple of "hey u y arnt u posting lolz!!1?" PMs. Since I can hardly ever bring myself to clean out my PM box as it is (because I'm lazy) invisible mode is an easy out.
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