While I've admired Flickr for awhile, I wasn't much of a user. Until now. Now I'm totally addicted, checking it eight times a day. What's so great about Flickr?
The most compelling part of Flickr is the social network. Flickr is not just photo hosting, it's a whole community system. And it's not just a social network site, it's a photo site. I share my photos with friends, they share theirs with me. I join groups. I look at strangers' photos and comment on them. I see what photos my friends like, and the photos of my friends' friends. There's always something new and a social context for what we're sharing.
The technical execution of Flickr is excellent. The UI is clean and fun, if a bit like a maze of twisty passages. There are RSS feeds everywhere. They have a great API; see Clay's notes for more. There's blog integration with badges and automated blog posting. And something I overlooked at first: updates happen instantly. You upload a new photo and it is immediately visible on every tag, group, and contacts page where it should be. It's not easy to make real time updates work at this scale.
Here's the addictive part. Flickr is a game, an MMOG * . There's the explicitly playful aspect, groups like squared circle and clock works. There's also an exploratory aspect to being on Flickr, where you go from photo to tag to photo to person to group to tag to photo, picking up images along the way and sharing them with your friends. And there's lots of interaction to keep you coming back, your recent activity and the group discussion boards. It's all quite compelling.