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.