I've previously mentioned Flock on this page. I use it to upload pictures to Flickr and to manage a couple of other blogs like bdfritid.dk and pfvejle.dk

I found the below mentioned piece of information on Techcrunch (yes its my favourite site) 

Mozilla has released details on The Coop, a new product that will incorporate social networking features directly into the Firefox browser. This is not good news for the privately-backed social browser Flock (also built on Mozilla), which is yet to release a 1.0 version of its browser. Many of the proposed features and some of the mockups created by Mike Beltzner (see above) suggest a significant overlap in the two products.

In fact, Coop even has an example screen shot of Flock on the wiki page describing the product, along with the description “The design will likely resemble [formerly of Flock] Chris Messina’s mockup for “People in the Browser”, with a horizontal bar containing avatars for a user’s friends, and icons overlaid on those avatars to indicate the presence of new content.”

The Coop product will allow Firefox users to “subscribe” to friends in the browser, bringing those friends into a sidebar. Those friends can share content and web pages with you (receive content from you, and send content to you).

Adding a friend will mean getting access to a broad array of their published web content. Content will be pulled from that person’s Flickr photo feed, del.icio.us tag feed, MySpace status , YouTube favorites, etc. When you want to share content with that user, you simply drag it into their avatar (see mockups below).

Source: Techcrunch and Mozilla