In terms of traffic, I think Facebook is the best social network today. You can promote your sites there by posting in various FB groups and pages.
Facebook - is the best way to stay in touch with friends and Fam. It even has awesome Business applications as well. Twitter - is the best way to post small bits of content to large numbers of people. MySpace - is pretty much worthless unless you're a musician in which case it is the dominant network. (most music venues won't let you play there unless you have a Demo CD and a MySpace page) YouTube - Is the largest video SocialNetwork in the World. This is where you shuld be posting all of your video content for business application, blogs. etc. Vimeo - is a video platform for Video content in the Arts. Short movies, creative projects,etc. (there are different user agreements with Vimeo that are IMPORTANT! You can get sued for posting copyrighted material on vimeo but you own the rights to your videos. YouTube can get sued if you post copyrighted material, but if you make a sweet vid and post to YouTube then they technically own the rights to it. It's in the fine print) StumbleUpon - Is like channel surfing the Internet. You can friend people and see what they like online. This is one of my favorite sites on the web. You can post your site to SU and if other people like it, then you get MAD traffic. This is really cool because people can find your site even though they aren't looking for it. LinkedIn - Is replacing the resume. It's where more than half of employers find new employees. If you're looking for a job, you'd better get one of these. Digg, Delicious - are both social bookmarking sites. If people like your site, then they can bookmark it on their Digg or Delicious page and other people can see what you like and possibly click to your web site. Sharelinks.co - (not sharelinks.com) is a place to bring all/any of these sites to one clutter free page that can be accessed from any computer with the internet. The best part about sharelinks is that you can instantly "share" your links with just about any of the above sites or even e.mail it to those who aren't on any social networks. It's free and probably one of the best tools on the web today. i hope all of this helps.