Use this tool to find your Flickr userid or groupid number. Just enter the address of your photostream or group pool and it'll find the number for you.
Twitter Tools This plugin creates an integration between your WordPress blog and your Twitter account. Pull your tweets into your blog and create new tweets on blog posts and from within WordPress.
BuddyPress is a suite of WordPress plugins and themes which extends WordPress MU and brings social networking features to a new or existing installation.
WordTwit is a plugin that utilizes the Twitter API to automatically push a published post to your Twitter account as a tweet, using the blog domain as a short URL.