If it was any of the above, you were WRONG!
Flipboard has risen up as THE summer social networking place to be!
"Flipboard is on a tear, adding 15 million new users to its platform over the last eight months. With 20 million users now reading stories on the mobile platform the company has managed to quadruple its user base in less than one year." ~ James Johnson
Flipboard's expansion plans very likely factor into the rapid growth pattern: people have been clamoring for more - as you can see in the small screengrab below -
PC Magazine: Google Announces Partnership With Flipboard