I will use G+ because I like concept & use of circles. I have more 500+ friends on FB and I am not able to update my self with my real friends.
If on FB anything similar to circle please let me know. Lets say I want to read updates only from my school friends or IT Milan member or PLUG member. Currently I prefer "List" concept on twitter. I prefer to read only twits which I think are informative for me.
I think Google Plus (G+) will stay here & will give good fight for FB. But it wont beat FB. I use Buzz as my primary two way communication social networking. Though I have account on 20+ social networking sites and 3-4 blogs. I like small community/network which is active around me on buzz, on which I daily spend more time than Twitter or FB or Posterous. I as actually very excited with Wave but it was difficult for my small brain ;) But buzz catch very well. Overall I am big fan for Google & its products. I used my gmail for 40% to compare my overall computer use, which include 5+ mails ids, 20+ mailing list and 30+ newsletters. From there I compose my all blogs and post to blogspot or posterous and from there it get auto posted on FB, Twitter, Wordpress and 10+ social networking sites . Even some time I post my buzz or twits through gmail :D
So overall Google understand social networking, web environment, human psychology. And thats why "Google Plus (G+) will stay here".