Chatting on Facebook is an interesting part for me. But is chatting with unknown girls legal in Islam?

Who would know but you ?

How do you chat with unknown women on Facebook (or anyone unknown)? You HAVE to have them friend you first... and if you have done that then they aren't exactly unknown.

Moveen,
You need to find a forum populated with people of the islam faith. There are none here that can help you.

If the heart starts to feel with desire, or the topics being talked about are not proper, then it is not allowed.

Merely talking or chatting is allowed as long as the purpose is correct