From the website and COO:

On Fotki, use free unlimited photo hosting for blogs, enjoy the cheapest photo printing, participate in photo contests, find new friends by interest, and host albums of photos and oodles of videos.

Fotki does not require registration for posting photos, let alone for using the site for many other purposes. There are over 1 million active registered members but realistically we have many more users - Google Analytics show 5 million monthly uniques. We serve between half a billion to a billion pages a month with a traffic of several billion image transfers a month.