I did a small amount of testing the other week on a niche and was trying to promote traffic through Pinterest.
Thing is I set up a Social plugin on my Wordpress blog which automatically posts to StumbleUpon, Facebook, Twitter, and some other random journal websites etc...
Before anyone asks, it was this plugin:
WordPress › NextScripts: Social Networks Auto-Poster « WordPress Plugins
Just a couple automatic posts to StumbleUpon over a couple days and it was obvious how powerful this site can be for traffic, especially since users can choose to follow all of your posts if they like what they see.
If you're not using StumbleUpon, you should really consider starting to.