Dude! Joe's Jottings, Mostly Junk

Google+ Growing As a Social Backbone - Slashdot:

Why is Google+ growing so quickly?

Because everybody on G+ only had to push one button to become a G+ user because they were already on Google in the first place. If you had gmail, google reader, picasa, google calendar, google documents, any google group, you were already in the unified system.  People don't have to "join" google the way they had to join facebook because they were already there.

If anything, G+ adoption is probably slower than it could have been.