I think it's an interesting idea. I track about 200 feeds, but probably look at only a small percentage of the stories. I definitely would appreciate help in blocking out the least interesting stories.

That said, limiting the free account to only 3 feeds makes it difficult to judge the site's usefulness. I guess that I would probably want to try the site out with at least 50 feeds to see if I wanted to pay for the ability to track even more feeds.

Consider that it may be more valuable to give out fewer invitations but raise with numbers of free feeds per user.