Rather than aim for just 5, you should look to take advantage of as many as you can, now that they're value isn't as great if it's just one, and from one singular source. Here's a blog post I found on the top 15 ones, along with some that I didn't even know about. http://www.ebizmba.com/articles/social-bookmarking-websites
@anil2009, I don't think so twitter is a social bookmarking site. It's a micro blogging site to share your interest in 140 character with your followers....