FB can be dangerous for that very reason. Unfortunately, I know too many first hand stories of affairs that started just like your innocent chatting.....
You didn't cheat, so no need to feel guilty.
If it was 1997 and he had called you to chat, would that be ok? Why then do people think it's ok to chat with someone on FB that they normally would not have contact with because it is technically improper? It's fun to talk with old friends on FB, but they probably wouldn't like it if I was chatting with their husbands too.