Is it a good idea (or how useful it is) to show a number of unread messages for each conversation when user enters page with the list of conversations? Would it be enough to just indicate that certain conversation has unread messages or it's better to also show a specific count for particular conversation?
Skype utilizes this feedback to the user, and I find it to work very well.
They divide the recent history feedback in two different stages. As the image shows, the number 2 on the Recent tab indicates that there are new messages from two different sources, individual contacts or from a contact group that you're part of.
And when you enter the Recent tab the application lists the two contact sources (only one in the image, didn't find a suitable one..) that has contacted you and for each contact in the list the number of unread messages is displayed right next to it.
Personally I find this to work very well especially to keep up and keep track of the conversation groups I'm part off. I'm not very interested in the exact number, however I am interested in whether 10 new messages has been posted since yesterday or 500 messages has been posted since yesterday.
The ammount will tell me two things, whether something interesting has happened that could be the reson for the large flow of messages, and also how likely it is that I will be able to backtrack the message history. 100 messages is do-able, 500 not so much.
It depends of what type of conversation it is. If it's the number of unread SMS-messages, I would say a counter is very valid. Someone is really anxious to reach you. But if it's a more casual conversation, such as Facebook chat - it really doesn't matter. So (again) it depends on the context.
But you can always leave the option to the user... do they want to see (1) the number of messages, (2) the number of unread messages or (3) new activity (regardless of the number of unread messages)?