If you obtain your content right, societal media sites can send 10's of thousands of guests to your site in a small space of time. But how much traffic do these social media sites actually send?
I have been communication to a couple of friends and looking at some of my client stats and have get nearer up with a rough guide to how many unique visitors on average you can be expecting to get from a popular story on some of the top social tagging and news sites. I have worked out my estimates mostly from Google Analytics.

Social news site Digg is famous for crashing servers, this is mainly because of the volume of visitors it sends in such a short freedom of time when a submission 1st hits the frontpage.
Providing you have some crash news or a story that will attention the community Reddit is a sure obsession for a quick burst of visitors. The amount of visitors a popular link receives will depend on how far up the frontpage it goes and how extended it stays there for.