Don't get me wrong here, I do that not because I have a lot of spare time in hand, but its because my significant one is a working nurse and a student, which makes her really busy. I am a developer/engineer and in front of a computer screen most of the time, which gives me some time to play around on online stuff, thats all. About facebook, we sometimes share the laptop at home and I don't care to look whose session is active on facebook, so I comment/like inadvertently being her, thats all.
Once you are married or in a long term relationship, you will understand that that there will be no "I" but "We". 

Though it wasn't necessary for me to explain it all, its just to clear the confusion :)

Cheers.