I can just see this girl sitting at her computer stalking your fb, then typing this out and sending it to you! SAD SAD SAD!!!
Past is past, I would prefer NOT to know who my OH has been with in past and I certainly would not ask someone if I had a suspision, id feel such a tw*t!
I would say ignore, but i'd find it hard to - id probably send back a message asking why is she asking you and not her partner?!