I know. This is not a new topic. People have been debating since Google+ launched. I have FB account & visit everyday, I also have Google+, but rarely look at it sine only one friend using it, who is my Fiance?!

My two cents of comparison---

Users number---
Facebook: 800 millions
Google+: 40 millions (given it only launched 2nd half 2011, this performance has been amazing)

User gender---
Facebook: Equally distributed, Male 51%, Female 49%
Google+: Male skewed, Male 71% (I bet they are all someone a bit geeky like my fiance), Female 29%

Group---
Facebook: Openly public, everybody can see what's happening in someone's life if they were blocked from seeing the 'update', however you can still go to that person's page to check the status, unless you are totally being 'unfriended'. So Facebook is a good place for stalkers
Google+: enables users to organize contacts into groups for sharing across various Google products and services. Users can share different information, etc. with different group, with different tone & manner, etc. It's easy to organize private meetings/catchup/event, other 'friends' wont' be able to see it. This way is better than Facebook, since you wont' openly state who you like and who you don't like.

Privacy---
Difficult topic, since there won't be privacy whatsoever for users. All the information will be disclosed to advertisers at various stage. Both sites have different way to segment the users, which make it easy to sell advertisement. In this digital age, people can't complain about the privacy too much unless it really physically harm your life. Because it is us who put all those information on the web for others to see, to watch, to critique.

Function---
Facebook: been evolving, with 'like', 'video', competition, so many different apps
Google+: ride on existing Google tools & applications, and more new features, like "+1" button, users can explore and empower themselves over time

The lists can go on and on.

I believe these two internet giants will coexist for a very long time, sharing the usage and love from users around the world. Win or lose? What is that so important?!