No matter where you are, here (SOF), in a chatroom, out in the world, remember one thing: If you have knowledge (i.e. you're a scholar) and you can reply then do it, but if you are not then learn and read first.
Also in Islamic Science there are priorities, for example you should learn how to pray and live like a Muslim first, then you learn in depth of other divisions like (how to argue with non-believers).
So in brief, learn and read first, cause if you don't know what you're talking about you'll do more harm.
Please note, I'm not a scholar, this is not a fatwa rather a simple answer to a known issue.