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.