Facebook has come up with a useful bookmarklet which allows you to find all your Orkut friends who have already moved to Facebook.

Facebook Friend Finder  

Simply drag the bookmarklet to your browser’s bookmark bar, and click it once you’ve logged in to your Orkut account. You’ll be redirected to Orkut ’s mobile version (m.orkut.com), as the service doesn’t work with the main Orkut site yet. Click the button once again, and Facebook Friend Finder will notify you of all your friends who’re in Facebook, based on their primary email addresses. You can quickly all / any of them to your Facebook friends list.