I've had a Flickr account for awhile now (I sort of "grew" one when Yahoo bought the site) but didn't really start using it until earlier this year.
One of the things I liked about Flickr, because it is such a popular site, is it has worked out deals with several blog hosting sites to allow you to post your Flickr photos from Flickr right into your blog. (This is how I was able to post my mash-up in the previous post). It's a nice addition, so that after you've uploaded photos you don't have to go back to the Flickr site, find the URL of the image, create the HTML to post the image, and then return to your blog site to post it all.
There are several different kinds of "groups" on Flickr. I have never signed myself up for one, though I was contacted by the 501st group - a Star Wars costuming club. We had the members come to my branch last October for a program. I had posted my photos on Flickr to share with friends and co-workers. One of the 501st members found the post and contacted me about marking them to be included in that groups album. That was the first time I had done that. It's a very cool way of sharing photos.