Ulbricht was grossly incompetent, they didn't need a special attack on Tor to unmask someone who asks questions about connecting to Tor from a public StackOverflow account in their real name.
IIRC they were able to track down the first mention of the site and it was from an account tied to Ross.
Plenty of noncriminals are Tor users and post about Tor - myself included.
> News reports linked a Firefox browser vulnerability to a United States Federal Bureau of Investigation (FBI) operation targeting Freedom Hosting's owner, Eric Eoin Marques. In August 2013, it was discovered that the Firefox browsers in many older versions of the Tor Browser Bundle were vulnerable to a JavaScript attack, as NoScript was not enabled by default. This attack was being exploited to send users' MAC and IP addresses and Windows computer names to the attackers. The FBI acknowledged they were responsible for the attack in a September 12, 2013 court filing in Dublin