Fifty Shades of Grey reached 35 million people more quickly than Instagram did

Earlier today I saw a stat quoted (with no source) on Quora:
"50 Shades of Grey grew to 35 million 'users' more quickly than Instagram did."

I looked in vain for a single source myself, so I decided to try to verify it myself:

Instagram:

The Next Web has detailed the growth of Instagram from it's start, taking Instagram's own reported numbers, since it's launch in October 2010.

"Oct 13 2010: 100,000

Oct 20 2010: 200,000

Oct 28 2010: 300,000

Dec 21 2010: 1 million

Feb 01 2011: 1.75 million

Feb 15 2011: 2 million

May 03 2011: 3.6 million

May 26 2011: 4.4 million

Oct 31 2011: 12 million

Dec 5 2011: 14 million

Jan 24 2012: 15 million

Mar 11 2012: 27 million

May 1 2012: 50 million"

Fifty Shades of Grey:

Yahoo! quotes a figure of 'nearly 40m sold' on 7th August, while the publisher reported 12m sales in the UK alone.  Since it was originally published on 20th June 2011 (Wikipedia), this means that this has happened in less than 14 months.

14 months for Instagram was at December 2011, so at the same point since launch Instagram had only 14m-15m 'sales' (it's a free app).

So yes, Fifty Shades did hit 35m people a lot quicker than Instagram.

Except that...  The 50 Shades figure is for 3 books, and I'd expect a lot of people have more than one of the trilogy, so it might have reached fewer than 20m people...