Sometimes I do. If I am at work and want to check in on my PMs or one specific thread I will be invisible. Also if I am busy working at home on something but just want to look in once in a while I will too. Most of the time if I am invisible on weekends or time off from work, it is because I am bummed about something and just want to be able to read in peace. I usually have to be very bummed for me to do that.