Imessage does not show contacts list in messages. By default when you open the imessages application it shows the sent / received window. This is ok if you are only going to send and receive to imessages contacts as you can search them from the search bar. It does get a little confusing if you have the same contact in many services. eg if you have Facebook or google talk connected into your imessages.
How to show contacts list in messages app for osx
iMessage > window > buddies
This will show the contacts lists for each service. This is almost like the way that iChat used to show the buddy lists. You can get a single list the same way as ichat if you have ticked the ‘show all my accounts in one list’ option. To set the single list option goto
messages menu > preferences
In the ‘General’ tab, there is an option, near the bottom for ‘Buddy List’.
Tick the checkbox next to ‘show all my accounts in one list’. Now you have a buddy list like ichat!.
You can also press
cmd + 1
To show / hide the contacts windows
You now get the old iChat style windows for each of your services. You will get one window list for google talk and one list for Facebook, etc.
To merge all windows into a single list then please see this post
sources: apple support