Facebook will become more and more large - with better interfaces.
Twitter will die when celebrities finally start to get bored - which is never, so Twitter is immortal, but it'll never progress, because the designers have lazy schemes and they aren't nearly as good as Facebook.
Youtube is at dying stage, they're just waiting for VEVO to finish the be-siege on them.
I think Google+ will probably be an advancement, although it's took it's time coming to the general public.
Hopefully, there will be an epic social-networking website, but hope is dry, because Facebook is always stopping social-network websites from prospering.