Flickr is a photosharing website that incorporates social media and many things "web 2.0" into how it works. It has become widely popular, especially among savvy users, due to it's design, functionality, embrace of open standards, and lack of restrictions.
Social Features
Folksonomy
One of the primary things that makes flickr a great social site is its folksonomy, or tagging system. Each image is tagged with words or sets of words that describe some aspect of the image, the content, the place, whatever the photographer wants. Viewers can also add their own tags if they feel something is missing.
Gather with others
Groups are a very important part of flickr as well. Any user can create a group which other people can join. There are two primary reasons for creating groups. To share photos on a particular topic and to discuss a particular topic. Each group gets a pool where they can submit photos from their own account to the group's shared pool. Pools are especially important because they encourage easily visualized participation. Each group also gets a discussion board where group members can talk about topics, organize, etc. It is a bit like a blog, except it's group specific.
Discussion Encouraged
Flickr also lets users comment on and discuss nearly everything. Every photo can be commented on by registered users. Sets and collections can also have comments on the set themself. Photos can also have areas inside the image selected and commented upon by other people. You can put a box around the bee on a sunflower and let other poeple know that it's a particular kind of bee.
Rich Permissions
Every aspect of flickr offers a high level of control over content. You can change the license photos are under (copyrighted, creative commons). Permissions for who can view photos can be controlled. Nearly all aspects of groups can be controlled as well: additional admins, limits on who can add to the pool, start new discussions, etc.
Extend with APIs
All of the functionality above is available through the Flickr API. This makes it ripe for mashups and other uses of flickr. See more in the examples section. Using the API, photos and related info can easily be incorporated into other web pages, not to mention all kinds of other projects. The APIs are extensively documented and well supported, meaning it is an easy to use
Why Flickr is great for Museums
Flickr isn't necessarily suited as a content mangement system for a collection, but it does work great as a place for documenting ancillary activities that might be happening in an institution , or by people interacting with an institution in some way.
Every museum should have a group of their own and encourage people to add photos to it.
Why Flickr great for Museum Blogging
Flickr makes it really easy to store photos, and does some of the leg work of resizing photos. It can help you cut down on bandwidth if this is a concern for high-traffic blogs. Flickr lets you post images that are hosted on their server, but requires that such images link back to the original image or set on their site. When you post images from Flickr on your blog, it is always a good idea to let people know that there are more on flickr. When you see any photo on flickr, click "All sizes", and if it's your account, flickr will give you the code to drop into your page for the size you've chosen.
Flickr aslo lets you post to a blog directly from flickr, using standard AP Is? that are part of most blogging software. We've found the layout options a bit restrictive, but it can work as a good starting point.