Just let it slide. You don't need to explain yourself, and you don't need to engage in a catty facebook fight. It's a crappy comment, but she's not your friend, not an element in your relationship, and if both of you were okay in you not going then it doesn't matter.

And really...I'm sure probably more than just her asked themselves why he is there dateless (it happens, people wonder...whatever) they just didn't post it on facebook.