i vote for facebook...but it's getting more and more ads now...i've never tried orkut, some day i will do it
I vote for Friendster, it may not be as popular worldwide but it's extremely popular in the Philippines.
I was active with some of the social bookmarking sites in the beginning of my webmaster career but now I no longer log in to these sites. I consider them not useful...
Personally I really like Myspace. I do have a Facebook profile but have not updated it in a very long time. I hear they are changing a lot of things so maybe I will check them out again.