I've seen it happen before that people gradually lose interest in a site's chatroom... it's sad.
A problem I think is that sometimes people check to see whether there's anyone in chat and then don't bother entering if there isn't or is just 1 or 2. Obviously this would mean noone ever enters and stays on their own.
Another problem is technical: if a person logs out of chat, then your pm with them disappears. If your connection goes or worse, the chat logs freezes for you, everything is lost. Chat shouldn't be like that...