Yes, the DOJ subpoenaed Stack Overflow as part of the investigation. It's pretty standard.
Normally the DOJ gets access to all the emails of the target of the investigation, then from there they look through the emails and subpoena any companies that might hold additional information - such as Stack Overflow.
Normally the DOJ gets access to all the emails of the target of the investigation, then from there they look through the emails and subpoena any companies that might hold additional information - such as Stack Overflow.