I guess facebook is a lot more convenient on features, simplicity, and connectivity.
orkut seems popular only because of the google brand name.
well, if you observe: orkut was the most popular site at brazil and india alone. the rest of the world were on facebook and previously myspace.
and also, in todays india, most people are switching over to facebook because of the newer games, features, network, and speedier connectivity plus the added option of most mobile devices being facebook enabled.
you also have a few popular sites that allow you to login with fconnect (facebook linked login)
therefore, facebook wins as a better social network website