IMO Facebook is not in any great danger from Google plus. Smaller start ups maybe, but Facebook has the not-tech-savvy market by the balls and google plus certainly isn't going to take them away. My brother, sister and father all use Facebook, none will be switching to google plus unless all their friends do, their friends won't switch unless their friends do... the Myspace/Facebook comparison doesn't work either, a…
StudiVZ is a very competent and very early (launched long before Facebook entered the German market) German Facebook clone, it has some nice features former StudiVZ and current Facebook users are still missing.
Everyone and everyone's friends were on StudiVZ. Now everyone is on Facebook (everyone is also still on StudiVZ but it's not really used). The transition happened gradually but all it took in my social cirlce were some people to try it out and invite everyone else. The action shifted gradually from StudiVZ to Facebook in the following months.
The same hasn't yet happened to Google+, at least in my social circle. (It also took Facebook a few months, I would comsequently not read much into that.) But it might very well. The StudiVZ story leads me to believe that switches from one social network to another aren't as impossible as it seems.
(Replacement is also only one of several scenarios. Co-existance is possible, I think.)