I tried clicking a kiss billboard ad on the forums, and the kissing link (which is an option when you buy a billboard) doesn't seem to work (https://subeta.net/kiss.php?user=USERID)

"Oh no, you've got an invalid link! Make sure to use a link from the forums, or use the fancy tag in a post!"

This is happening to me as well. At first I thought it was my billboard but after clicking several others, I ran into the same problem.