Knol's homepage says a lot about the current state of the project. There's a big empty section called "what's new", a single featured knol that has 1,000 views, while the "most discussed" section doesn't include any knol and the search feature no longer works properly.
Knol has been last updated in December 2009 and it's obvious that the service has been abandoned. Somebody needs to close Knol before it's too late.
If you want to find a list of abandoned Google products, check the copyright notice at the bottom of their homepage and see if it's been updated. Friend Connect, Google Desktop and Knol still show an old message: "©2009 Google".
Update (one day later): Knol is up and running again, but for how long?