According to Media Culpa a blog that apparently obsessively tracks these things, photo-sharing site Flickr has hit the 5 billionth photo milestone by Sept 19, 2010  with the above, uploaded  by Flickr user yeoaaron.

Update: Kullin shares some of the methodology behind tracking the milestones.

Flickr gives each photo a serial number and they have previously officially confirmed that for example the photo with the ID 3 billion is actually the 3 billionth photo: http://blog.flickr.net/en/2008/11/03/3-billion/ Date and serial number of a photo can be found via Exif daya:http://www.flickr.com/photos/lenglengtan/250000000/meta/.

Previous milestones for Flickr:

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

Despite the fact that social networks like Facebook have become popular places to share photos, it seems that the growth of Flickr continues at the same pace as during the last 3 years, adding about one billion photos per year.

via  Media Culpa