Since the reason for the variable degree of success of online social tools for scientists is largely attributed to the lack of participation, I think a great way to pull in participation by scientists would be to offer that kind of value up-front. You give it a paper or set of papers, and it tells you the ones you need to read next, or perhaps the ones you’ve missed. My crazy idea was that a recommendation system for the scientific literature, using expert-scored literature to find relevant related papers, could do for papers what Flickr has done for photos.