There is a great deal of websites that are going through several issues. Some are slapped by change in algorithm, other are crying for blocking Google by mistake, some are hacked and some manual action penalized. The curious case of websites is those for me that are not hit once and for all but are killed something like a slow poisoning. There traffic dries out on constant rate.
You might have experienced a great deal of such websites yourself and there is always a thread in Google Webmaster Help forum for their help every now and then.
Here is a graph regarding the working of such websites and the kind of traffic they receive over the time.
I am sure that like every sensible SEO expert you might find yourself haunted by the question that what really brought the website to such low levels. Its surely was not a specific algorithm change or a typical issue with not complying with Google. One might guess that may be the competitors are acquiring higher rating causing this one to go lower. May be Google is losing trust over the site with every day passing and that’s why thrashing its traffic and ranking. It is also possible that site was removing links from the sharing partners making it to lose its organic search traffic. It is highly likely that they might have lost the interest of users because it was only useful some time ago and now it is totally waste of time.
Have you really seen such sites? What would be your suggested course of action for them?