I'm sure that Facebook is used as a 'list maker' for the gov't. Zuckerberg ILLEGALLY gave 0bama unfettered access to FB profile data for his organizers to use for the 2012 election, so I'm sure the DHS, FBI, CIA, IRS, etc., builds lists based on people's 'Likes', etc., for compiling lists of those who are potential resisters to the gov't ("domestic terrorists" as Napolitano once said).
Who knows who. Who belongs to which organizations. Who has what interests.
There’s simple apps which produce a network graph
http://www.touchgraph.com/facebook
and for professionals more sophisticated social network analysis software some which can automatically extract data from facebook
https://en.wikipedia.org/wiki/Social_network_analysis_software
This article is an informative introduction
www.kieranhealy.org/blog/archives/2013/06/09/using-metadata-to-find-paul-revere/
So is Free Republic. We've all been on 'the list' for quite some time, so it doesn't make a damn bit of difference if you're on Facebook or not.