Facebook chat was notoriously awful from the beginning. Besides being laggy and crash prone, FB chat lacks even the most basic IM necessities-- like the ability to telegraph whether or not you're actually available to talk to anyone. On Google+, what you're getting is the far superior Gchat-- complete with all your contacts from Gmail.
But the real killer app here is "Hangouts," a new kind of video chatting service that Google invented. From Google+, you can go to the hangouts page and see which of your friends have webcams and are available for a video conference. Only people who have said they want to hang out show up on the hangouts screen. Up to 10 friends can join a hangout at once, creating an unprecedented virtual get-together. Facebook recently rolled out video chatting via a Skype partnership, but it only works 1-to-1.