With all due respect, an IP address can also be assigned to your wireless router, and multiple devices can be logged on simultaneously. MAC addresses are assigned to individual devices, but they ARE accessible, by google, and LE.
There is a current case involving the FBI regarding this subject. However the redacted affidavit contains such disturbing details, I can't link it. The complaintant was able to enlist the FBI's help after receiving these messages on a social networking site - and the MAC address was traced.
Besides, with a subpoena, and the inherent make-up of the OSI model, this makes sense. You'd be surprised how much data and metadata are stored, ESPECIALLY by google, aka "Scroogled."
Do a search on "badgerherald.com FBI Special Agent Malia Pereira Ragsdale" if you want to find the affidavit PDF.