From the Mental Floss site yesterday, “Why Have People Started Asking Questions by Adding ‘Y/Y’?” by Gretchen McCulloch:
This is the best new way of asking questions, y/y? Some examples from around the internet show how this method of appending a y/y to the end of statements is starting to be used.
So I should wear my matching shirt at some point, y/y?
So, Ramsay is the new Joffrey but 1000x worse, y/y?
#knitting friends. We should all make these for next winter, y/y?
I have a million of these flower dresses and I need another one y/y?
the weirdest hat he’s ever worn, y/y?
This is strictly an orthographic feature; y/y? isn’t an abbreviation for spoken yes or yes?, as McCulloch points out. Instead, it’s a very compact way of converting a (written) statement to a question seeking agreement with that statement.