Comment 2 for bug 1187425

It should show all unread messages in my opinion and mentions of your nick in a second separate badge.

Something like this:
#elementary 10 2

Where 10 is the count of non-mentioning messages and 2 is the count of the ones that do mention you.

This could also be displayed in two separate colors like Plank badge color for messages that mention you and another color that fits into the gray theme for the normal ones in my opinion.