Almost every site now (including this one) has buttons to share the page or subscribe with specific web services like digg, google reader, delicious, etc. I've personally never clicked one (I always use browser extensions or other methods for the services I perfer) but I assume that lots of people do given these buttons' popularity. I run a personal blog and currently just have a vanilla link directly to my atom feed. I'm considering adding some buttons to share/subscribe to web services but I don't want to clutter up my UI with stuff no one is going to use. Are there statistics somewhere on what the most popular services are? There are dozens of these sites and I'm very uncomfortable with adding a button for every one.
I would add Facebook and Twitter.
To find out what works best, check a popular news website or related blog and see what their usage is. Check out a few posts and be sure to notice the topic; different demographics may give different results.
Example: The New York Times makes Facebook a featured share button for general news. And, if you go to a tech article you have Twitter as well.