FB has better quality traffics, almost all visitors from FB to my site stay a while or at least browse few pages. Stumbleupon is the worst, 99% just stumbled my site for few seconds and leave, they jump from one site to another fairly quick.
If you want traffic to your website, I would highly recommend you focus on good long-term SEO development. Aside from that, the social networks I'd recommend focusing on: Digg, StumbleUpon, Twitter, Facebook. If you can get popular on Digg you will receive countless relevant backlinks (great for SEO), along with thousands of unique visitors (who may thereafter become regular users of your website). StumbleUpon is great for long-term traffic. If you can get your content to do well consistently, before long you will be getting hundreds of visitors regularly without having to do anything. Facebook/Twitter are just a given. Build up your networks, stay connected, post good content, and wait a few months = success!
From professional experience I would recommend Facebook. To increase your traffic there however - you should pay attention on how you manage on daily bases your company fan page. You should have in mind that a Facebook page is the place to prepare and manipulate users to search carefully for your info when they land on your website. Something more - it can pull users mails, its interactive - you can be a part of the society of your future clients, you can challenge users by games and awards on your wall and its the strongest tool of active online marketing ( as it brings clients to your website by pulling them from the biggest social network) A good way to attract attention as well is how you brand your page - a nice way are the additional buttons with dinamicaly changing thematic designs - linking to your website places We (at ooyes.net) have worked it out for a lot of our clients
Facebook is better for reliable traffic if you have a fan page. Twitter is better for viral traffic so people RT your posts.
My votes goes to Twitter, bcoz you linked with twitter may social bookmarking and article sites .......... so it's easy way to share your link many other sites ..........
In terms of getting only visitors none better than Facebook. Twitter is very close to Facebook. I don't think Orkut can bring you that much of traffic.
One more vote for Facebook. Then Twitter. But with any of these, the key is to learn how to actually do it...