Lynne Kiesling points to an interview with the creator of Digg.com, Kevin Rose. I found this part especially interesting:

Already you can get recommendations from friends, but soon the system will start recommending stories that you might have missed or that you might find interesting, based on what you’ve dug in the past.

That would add a pretty substantial private good onto the existing public goods provided by Digging, because Digging interesting articles will improve the quality of articles Digg suggests to you. Adding a private good should internalize the benefits and increase the supply of Diggs. I don’t use Digg right now, but I’ll be sure to start when this comes out.