What is this "pwstdfy" parameter showing up in Facebook URLs now?

This might help..
http://mashable.com/2007/07/31/facebook-downtime/
Check the comments..

From: CS in the comments
Most likely they had a bug (thier code probably spewed the wrong Cache-Control/ETag/If-Modified-since or related http headers) that made proxy servers cache and display the contents of someone else going through the same proxy (reading other's inbox etc).

If you notice every facebook URL now has the pwstdfy parameter that has been added to make the URL unique (hence not be cached by the proxy servers). It will probably stay there until they feel the content has now been flushed out of the proxies.