While this is not domain related, I found an email that I received by Facebook yesterday very interesting.
The phone number 1917xxxxxxx has been deactivated for Facebook text messages due to the following reason: the mobile service provider this number was registered with no longer recognizes this number as valid. If you changed your mobile number or switched mobile service providers, please visit Facebook to add your number again.
The Facebook Team
I understand that the number is needed for their messaging service, but I had not used the messaging service since I changed the number, so I assume that Facebook is monitoring all changes. It made me think what other things that Facebook or other services that I had given information to in the past is monitoring.