This annoys the shit out of me. I have three friends on Facebook (people that I know personally - two family members and one old friend) who have their boyfriend's last name listed as their on.
Don't play wifey if you aren't married. And if you don't feel the need for "a piece of paper" or "vows are just words" then fine, but don't parade around pretending to be something you aren't. Shit's worse than wearing a big ass CZ wedding ring. Fake! (which is an entirely different debate...).
I take pride in being married. Marriage is a holy unity to me. It isn't something I take lightly and neither should anyone else. So for a 16yo who's knocked up by her boyfriend and living with his parents to carry on like she's married... yeah, *eyeroll*. Or for a woman who's been living with her baby daddy for 3 years, but the man won't actually propose to her, to wear his last name like she's married. Ugh.
I am proud to call myself my husband's wife and proud to have his last name.