The growth of Flickr

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"22 Oct 04: 1,000,000
20 Apr 05: 10,000,000
15 Feb 06: 100,000,000
22 Sep 06: 250,000,000
15 May 07: 500,000,000
19 Jul 07: 850,000,000
06 Oct 07: 1,500,000,000
13 Nov 07: 2,000,000,000
17 May 08: 2,500,000,000
03 Nov 08: 3,000,000,000
04 May 09: 3,500,000,000
11 Oct 09: 4,000,000,000
18 Sept 10: 5,000,000,000
1 Aug 11: 6,000,000,000"
Source:  Blog post on Kullin.net, 9th August 2011