Sorry to jump in on an"old post", but I agree and disagree. I think they can exist because they will serve different purposes for folks (like Ryan is saying) but I think it will be longer. Think of it this way, myspace is still around and very active for a certain population. Both of these will co-exist because you will have a population that is ready for something beyond FB. Something beyond 420 characters. Something beyond the endless new games and new game update posts you can't get rid of even when you try. Something that doesn't opt you In to automatically when it loosens privacy rules. Then you will have others who will want the granular control it appears that Plus is offering. That combination of services to bring many of the tools they already use into a social scene without leaving one interface. Etc.
Gravity and familiarity will keep others at FB. For some FB perfectly serves his or her needs. For others it is a way to stay away from Google,,, others will be too invested or not want to learn a new system. Some will dabble and "not get it" or feel they do not want google crawling through more of their information... It is hard for me to wrap my mind around 3/4 of a billion people suddenly moving from FB to Plus. And for the first week it was pretty lonely here as few were in to actually interact with. Now that has been changing and there seems to be more interest.
I also like the granular control of sharing on plus and the ability to easily "discover" new perspectives through "nearby" and public posting for some information. Sort of twitteresque in a way and in the way you can have people circle you who are not in one of your circles... It will be interesting to see how this develops over time and with the impending launch of iCloud. What impact will that have. One thing is for certain, to really penetrate Google needs to get the iOS apps approved. That alone will keep folks from even investigating Plus if it doesn't happen. I am ok looking to jury-rig my way around to do most of my interactions mobile, few others will sustain that for long.
Just my thoughts,