I've clicked on those ads that they have now already. And since they were interesting things, I'm likely to keep an eye on that area for the foreseeable future, to see if anything else neat crops up.
Seems like an excellent way to roll-out ads... first, make the users look at the area where you'll place the ads, by putting interesting stuff there, and then put paid ads there.
So long as most ads remain relatively interesting/targeted, I'll keep looking. And probably clicking, too.
Consider also that Twitter users are very used to clicking on external links, because we do it all the time for links tweeted by others. I am much less likely to click on an external link in FB than on Twitter.
Facebook has some excellent targeting. I've gone to a few Facebook links and if I wasn't broke I'd have bought some products, too. (Right now they insist on showing me the Twilight DVD, though, which irritates me.)
http://www.itworld.com/business/64445/twitter-house-ads-appe...
HN has a lot of prime homepage real estate to the right, but if they put a link there to the submission form, it's hardly an ad :)
The standard way is to place ads and juggle them about until you've figured out where they attract the more clicks. Twitter's way appears to first manufacture a high-interest area (by putting cool twitter stuff there), then, once people are used to seeing nifty stuff there, start selling that nifty space to others.
Facebook already has ads, though, and they're evolving a business plan constantly. Twitter has nothing. So they have less experience with raising money and they've got more to lose.