Whenever someone will search your site typing your site related keywords,your site will show up.

This is the benefit of stumble upon

More friends in your network, the more traffic will come.. but make sure your blog and content attractive, informative and updated..

You can subscribe to a tag, either from a single user, or by anyone who uses that tag, it helps to have friends. You do this by adding people to your network. You can then share content with them simply by clicking on their names.
Having a strong network provides you with the best chance of making it to the popular page when you have new content that you submit.

You can also submit to digg and try blog commenting. Facebook and other social networking sites also helps in generating traffic by sending/posting messages to groups.

ya social media sites do help to get good traffic if used wisely.........

Unfortunately its not just a case of submitting your site and getting traffic - how much traffic is usually based on how much participation your account has with the community. StumbleUpon sends traffic based on how many recommendations you've made that have been agreed with by other StumbleUpon users, and also takes into account how many followers you have. Other sites such as Del.icio.us are more passive in the amount of traffic they send, but can prove a good metric to see how popular your site is in the social media sphere.

If you can build a network on those sites it will be more beneficial in terms of them sending you visitors.