From the (excellent) Social Media ProBook:
Flickr: The popular photo and video sharing site launched out of Vancouver in 2004. Acquired by Yahoo! just one year later, Flickr grew into an online home for more than 51 million users and 5 billion images. But as Facebook and mobile photo services like Twitpic and Yfrog add options to the photo-sharing market, Flickr has seen a decline in their audience over the past two years. The site is far from dead, however, as accounts like The Official White House Photostream continue to drive traffic to Flickr. In some ways, it’s returned to the purest version of itself – that is, a hub for photography enthusiasts. It never became the corporate marketing “outpost” some originally imagined.
A rather unexpected way to explore Flickr is the notorious Flickr Panda.