It's awesome playing games on Facebook with your friends especially since nearly all your friends are already on Facebook, but are they really on Facebook? Many of your friends play on different platforms like Xbox, Play Station, iPhone etc. We find ourselves in the predicament where we can only Game On with some of our friends.
A truly social game needs to be available on all major platforms/devices, and very importantly the platforms need to be linked/integrated. There are some popular Cross Platform games but you still can't Game On with all your friends because the platforms aren't linked. It is great to play a game on your iPhone on the bus but it is not so great if you can't Game On with your Facebook friends at the same time.
Cross Platform is about making a game work seamlessly on all major platforms, and linking all these platforms, allowing you to Game On with all your friend anytime and anywhere.