Ignore all the facebook angles, consider if someone actually wanted to build database of known copyright infringers (I refuse to use "pirate" since it isn't hijacking ships on the open seas)...

Scenario: MAFIAA would like a "high value target" database of IPs to check each time a new high-value release comes out.
1. MAFIAA have zero IPs today.
2. MAFIAA "front" puts out a "History of Torrent Downloads By IP"
3. "We" check it - and hand them our IP, which they add to the database.
3a. Whatever the site tells you back when you check is irrelevant.

When high-value release is made:
1. Pass IP database to associates who have the tools to port probe looking for torrent ports and query what they are seeding.
2. Pass confirmed hits IPs to the "Collections" dept for letters to ISPs and/or John Doe lawsuits.

Probing a few hundred thousand people who thought "crap, did they catch me?" is all of a sudden so much easier and your paranoia ironically leads to you being caught.