As a super premium: allow people to upload their own text and graphics for a dollar, and attach that to the tweet.
@squarejaw bought *victim* a pair of nads
@ironical sends you a touching violin solo on a miniature DVD
@citygirl sends you another pair of pink polo shirts to wear over all your others
"Are you joking?"
"I don't even know anymore."
I have also paid for software, music, videos, text messaging, online connectivity, phone time, concerts, priority boarding, and World of Warcraft.
But I wouldn't ever pay money for virtual goods, oh no, that stuff just isn't real.
(Moral of the story: it is all how you sell it.)
http://gigaom.com/2009/07/30/12-of-americans-bought-virtual-...
(hint: offer them in black and in XXL)
p.s. y combinator should have a "forgot my password" button...or if they already do, make it a little more prominent
Besides that you should get retweets/replies working, i wanted to RT a few things and got put off by the coming soon box.
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.)
They were in a similar situation and according to Wikipedia, they sold in 2008 for a rumored $20 million. They started in 2000 and I believe they made a deal with their internet service to reduce cost.
On a page like this, http://favstar.fm/users/asshuku, the Japanese kanji is being rendered like Chinese kanji. It's still readable, but it's just a little off-putting. I'm not native Japanese (I can read it, though), but the best example I can give of what this feels like for a Japanese person is that all of the text on the page has been highlighted and underlined. Readable, but annoying.
I was playing around with it in Firebug, and it looks like a simple(?) fix would be to change the line: <html lang="en" xmlns="http://www.w3.org/1999/xhtml xml:lang="en"> to: <html xmlns="http://www.w3.org/1999/xhtml xml:lang="en"> or: <html lang="ja" xmlns="http://www.w3.org/1999/xhtml xml:lang="ja">
You might also be interested in seeing Google's solution (http://www.google.com/search?q=google.co.jp), Slashdot's solution (http://www.slashdot.jp/), or Wikipedia's solution (http://www.wikipedia.ja/).
I suggest diversifying your ad real estate, at least initially, there's no reason you can't have Featured Users + Adsense + Deck, etc. Just don't go overboard with the # of ads on the screen, you'll drive people away.
You may also try using Google AdManager or OpenX and selling ads yourself. I prefer the former because Google handles the hosting.
1) Add tipjoy functionality, encouraging users to tip the best/funniest/helpful tweets.
2) charge referral fee to users who you scrape. Each new follower they get from favstar.fm costs $0.10 (seems like many of the people you are featuring are self-employed or creative types who would probably like more followers)
3) sponsored favoriters... allow brands/users who want their profile pic to show up as if they had favorited a tweet. pay per impression or click basis (they could get creative with the profile pics to grab attention)
4) promotion of top favorites across the web. build plug and play modules that people can add to their own sites, my yahoo, igoogle, their facebook page, etc. (or maybe just an RSS feed). Anyone can post it to their own site for entertainment value, but the only tweets that are eligible for the module are ones from paid accounts. Trouble is, the best/funniest twitter users probably wont want to pay for that distribution, they don't need it.
Another suggestion, although not monetization related - provide a widget of say the top favorited / top favorited users - that bloggers could embed on their blog - increases distribution.
You might want to check out Chris Anderson's book on Free: the Future of a Radical Price as a good source for brainstorming business models.
Forget monetization for now. Try to get acquired by somebody who think they can make use of your traffic.