In Google+, you can chitchat with anyone in your Circles who also has added you to their Circles. And, when you’re not using Google Plus, you’ll still be able to chat with them using Google products including, Gmail, the Google Talk client, iGoogle, Orkut and 3rd party apps.
What’s changed with Chat?
Previously you could only chat with someone if you knew their email address. Now you just need to have each other in Circles.
When you and your contacts have each other in Circles, you’ll be able to chat with them across Google properties such as Gmail, Google Plus, iGoogle, Orkut, and the Google Talk Client.
Instead of showing all of your contacts in your Google Plus Chat list, google plus showing contacts from your most recent conversations who are online now.