The good news is that diigo.com is not defunct.
The bad news is that their domain has been momentarily hijacked, see http://www.diigo.net/about/domain:
Dear Diigo users,
We’re terribly sorry to inform you that we’re experiencing domain hijacking, ie. someone gained access to our Yahoo domain registrar account, and illegally hijacked the domain, www.diigo.com. Very soon www.diigo.com may not be accessible to you until this issue is resolved.
But please rest assured that all our servers and user data are NOT compromised, and your data can be alternatively accessed at
Your current Diigo extensions and bookmarklets will not work on diigo.net.
For now, to bookmark to diigo.net, please install this special bookmarklet for diigo.net >>
Again, we’re terribly sorry about any inconvenience this may have brought you. We’re working hard to resolve this. Thanks for your patience and continued support.
For the latest status update, please see our tweets at twitter.com/diigo
The Diigo Team