Sponsored tweets are a good idea. But results are best when companies get to target the audience based on their interest. For example, a company would more likely to spend to show ads on specific search term than all the search term. So if you can find a way to categorize the people visiting your site based on retweets or fav patterns and have separate categories of favs then you can have targeted sponsored tweets.

Sponsored tweets sounds good to me too.

Indeed -- something a little like StumbleUpon's model, where a declared percentage of displayed items are actually paid placement, could work.

Payment could even be in the form of "virtual favorite points" -- ranked score is real favorites + bought favorites, though with some cap to prevent the list from becoming all paid.

(OTOH, 'favoriting' isn't hard to fake, is it? So as soon as being atop your list is economically valuable, it may become dominated by spam.)