In the memo that was leaked, USCIS states "Narcissistic tendencies in many people fuels a need to have a large group of "friends" link to their pages and many of these people accept cyber-friends that they don't even know. This provides an excellent vantage point for FDNS to observe the daily life of beneficiaries and petitioners who are suspected of fraudulent activities."
How do they do this?
They friend you and hope you will accept friends you do not know. Then they read your postings and look at your photos and look for indications you are having romantic relationships with someone other than your US "spouse" or not living in the marital home.If you are seeking naturalization, they may look for indications of criminal activity that you have not disclosed,like your posting about a speeding ticket or arrest,
Can they do this?
Yes, you have invited them into your site and it has become a public record otherwise. You have no right or expectation of privacy if you posted it and gave them permission to be your "friend"
What can I do?
Be careful about friending unknown people. Be careful about what you post generally as you do not know who will eventually have access to this. Just Be Careful.