Archived chats are limited to 100 transactions or to 20 minutes of chat - A transaction is counted each time a user sends a new conversation line or hits Return on Messenger.
Chat through instant messages
Having trouble chatting with your Facebook friends with Yahoo Messenger? Relink your Facebook and Messenger accounts .
Want to chat with your Facebook buddies on Yahoo Messenger? Once you link your Facebook account to Yahoo, you'll be able to chat with your Facebook friends in no time!
Are you chatting with someone on an older version of Yahoo Messenger? Certain features won't work when chatting with someone who is does not have the latest version of Yahoo Messenger.
This article explains why you might sometimes receive what appear to be chat messages in Yahoo Mail.