I'm a fan of Google+ and use it. It serves a different audience
  for a difference purpose. It's not a failure; its just "not
  Facebook". Facebook is all-purpose and general.

  That's its power. G+ is more about supporting people's fandom
  and obsessions. Facebook is getting better in that regard though
  and I give them credit.

  The "Facebook killer" probably won't come from an expected
  source. Google had its fame: The search engine and gmail. That's
  its two steady claims to fame.

  Microsoft has Windows. That's its claim to fame.

  Linux has Android; that's its success. [I doubt it'll find full
  success as desktop/laptop replacement, as neither will Chrome].

  Facebook has Facebook.

  But there's other social networks, serving other needs. I know
  quite a lot of people who don't have Facebook, either never
  having had it, or getting rid of it. You won't see them here.

  A good example is Steam. You'll rarely find any of them on
  Facebook. They're socializing in the games, chatting on their
  headsets.

  You have tumblr. You have deviantart and... of course... *sigh*
  - 4chan. Oh and reddit too.

  All social networks serving different needs. Many people never
  go into the land of Facebook.

  Maybe Facebook is becoming what email was; and if it has
  succeeded/will succeed in that task, then it could last
  perpetually but... I dunno. I believed in so many super-large
  networks through the years only to watch them crash and burn,
  it's hard to believe any of them will have perpetual staying
  power.