This has been done in the past: researchers visited a uniquely generated URL from Tor and then recorded which Exit Nodes visited it again. You can find their work if you google it..
"Chloe" visited unique web pages for a month last year, and also used unique credentials to log into a custom honeypot. Of the over 137,000 exit nodes tested, 15 attempted to use the credentials, 650 visited the unique websites.
Less than half of a percent, but definitely happening regularly enough to be an issue.
