this graph is extremely misleading as everyone with a gmail/youtube/andriod/google account has a Google+ account.
It puts into perspective how much of a failure G+ is and the main reason is the same reason google-branded phones are not the best selling phones -
Google doesn't market its products aggressively. They expect the public to just use them and realize they are better.
Honestly, they were extremely lucky in becoming the #1 search engine in the world. It was more about how bad the competitors were than how good google was.