I like having the option of multiple social networks, it's getting them all working together that is the challenge. Facebook & Twitter are the obvious top ones right now.
Facebook status to Twitter
You can use
Twitterfeed to push your facebook updates to twitter. Unfortunately Facebook does not want anyone to use Twitter so they make it difficult to find your RSS feed. Use this plugin to get it, and use the one for your posts only:
http://apps.facebook.com/rssexport/?ref=bookmarks&count=0&fb_source=bookmarks_apps&fb_bmpos=2_0
In Twitterfeed, make sure it is set to only display the title, or else it will show the update twice and look nasty.
Onesheet
It's simply a webpage that collects shit from all your social networks together in one place. But personally, I believe your official website should already be doing this. On the other hand, I also believe you should register and setup your stuff at every site available, I don't think it can hurt.
Posterous
Makes it easy to update your blog/website. You can do it via email or text message. Send pictures and stuff too, very useful, I would imagine this would be awesome while on tour to upload hilarious pics at 5am when your bass player shits himself after passing out again.
You have to find something that works FOR YOU. I update Facebook all the time, so my stuff feeds from facebook. If it's easier for you to update via your website, have it feed twitter and facebook and vice versa.
The most frustrating thing about the current state of social networking is that it is a pain in the ass for bands to network. That was what made Myspace so amazing, bands could book shows and talk to each other freely and it really helped things happen. Facebook pages for artists are just shit. I can't message anyone about anything, I can just post shit on my own wall and that's about it. Not effective.