Stack exchange has feature to chat with other learners/teachers available at chat . So is it haram to chat?
Because I think mahrams and non mahrams will be there.
The answer somehow depends on your behavior and your intention in a chat room.
Basically one is allowed to greet a non-Mahram and make any thing which falls under necessary parts of conversation with him/her.
So if you are chatting to seek knowledge or exchange some information etc. then everything is fine and following the rules of shari'a. But once you leave this line for example by doing chitchat or anything else you might be doing something sinful and it would be regarded as haram!