This post describes how Social Alerter (SA) works in detail...
SA tracks in real-time social bookmarking and social news websites (the list is here). While watching these websites, it looks for the websites you've specified for alerts. If it finds that a story is getting hot, it sends you an email warning you about the likely flood of traffic. This allows you to prepare your site before it crashes under a lot of traffic.
You specify which websites you want alerts for at the New Alert page. Simply specify the domain name of the site you want to track and you're done. Full details about how to specify a domain name are at the Tracking Domain Names help page.
At the moment, you can track an unlimited number of sites, but if you stress the system too much, we'll contact you to figure something out.
Finally, please read the Help section. It links to a lot of pages describing the Social Alerter service.