Android is totally pluggable - Twitter, Foursquare, Skype, Google+ all use this open API's for Android to intergrate themselves into the Android experience - The contacts appear in Android contacts, the pictures appear in Android contact and the status updates appear in Android contacts. With Android 4 this integration goes even further - meaning app developers can push status updates from their social network into a mini news feed style section in the contacts app. Additionally - any application that wants to share something - sends a share intent and a list of apps that accept share intents pop up (like SMS, Gmail, Email, Twitter, Google+ etc etc) Any app can register to receive these sharing intents.
Google+ is not really "integrated" on new Android 4 handsets any more than Google Maps is. In reality - Google+ is pre-loaded - and just uses the aforementioned API's that hook into the OS. The details of how to do this are publicly available. Any social network that wants to do it - can easily do it. However - as Facebook have consistently refused to do pretty much anything to make their app halfway decent on Android - don't hold your breath for them to change now.