Some of these statistics seem to be widely used. Due to several botnet takedowns by Microsoft along with law enforcement agencies, reduce spam amounts drastically for a few weeks but they seem to creep up again. (This article for example says Global spam 88% of emails, but spam fell by a third after the Rustock botnet takedown by Microsoft. The source seems to be MessageLabs, an anti-spam brand of Symantec. They use statistics from servers running their software.)
Another good source seems to be MAAWG, which use confidential data from ISPs, mail providers etc. to collect statistics on about 500 million mailboxes. They estimate spam from between 88 to 91 percent of email traffic.
MessageLabs and other Security-as-a-service providers are in an excellent position to provide these statistics. They all agree that in recent years the number has been above 80% almost continuously. Every spam botnet takedown temporarily reduces spam, however numbers rapidly increase as other spammers move in — see this article at The Register.