The survey also suggests that Google+ will likely chip away at Facebook usage and engagement:
As Google+ gains followers, it is likely to negatively impact Facebook usage. 30% of Facebook users who already use Google+ plan on cutting down time spent on Facebook this year.
Taking a step back, no survey can reliably predict the future. Very often attitudes expressed in surveys are quite different than behavior turns out to be. Accordingly we should be cautious about drawing conclusions about the fates of Facebook, LinkedIn and Twitter, at the hands of Google+, from this single survey.