A few days ago Stan Schroeder of Mashable posted a entry entitled Facebook Trumps Most Photo Sharing Sites With 10 Billion Photos. In it he says about Flickr that "ultimately people use Facebook for the same thing - sharing photos with their friends" and that "It’ll be interesting to see what photo sharing social networks will do to differentiate from Facebook".

It is possible that Stan still uses Flickr in this way, but I suspect many people already use large social networks (like Facebook) if they only want to share photos with their friends.

This is not a problem for Flickr, however. Flickr has established a community of people who want to share their photos with the world. Links to photos on Twitter or blogs often use Flickr for that purpose because twitter and blogs are by their nature public and Facebook is not. Through Flickr I have (re-)discovered photography, Flickr has a fascinating array of photos on it which can be easily found by going to the interestingness pages or one of numerous groups. Flickr also has a number of features which support it as a place to share photos with the world, not just your friends:

• Tags
• Public Groups
• The API and various applications built on it (e.g. bighugelabs)
  
• A community of photographers
• Geo tagging
• Search

(Facebook also has something called tags but that is for tagging people, which makes it useful for sharing with friends but is not the same as what Flickr uses tags for)

In short, Flickr is a way to share your photos with the world and Facebook is way to share photos with your friends.