Well, it is always "people how clicked on FB that they like X", regardless whether it is shallow or deep interest, genuine or ironic, or random, or "because my friends like it and I want to be as cool as them" etc.

If more fine grained, I wouldn't be surprised to see ties between seemingly exclusive things... e.g. in this http://meta.stackoverflow.com/questions/157976/map-of-all-se... (not interesting, but participation in particular Q&A sites) "christianity", "judaism" and "islam" are in the same category (as opposed to people apathetic to that topic).

That's not precisely correct. In the old days, we typed in our favorite bands/books/music. The attempted canonicalization created some amusing interpretations of song as band, book as author, band as book, etc.

It has also made it exceptionally annoying to get feed updates from the 900 bands, actors, movies, songs, etc. that I said I liked and now have to "unlike".

I think you could correlate people within a certain confidence, but because of the nature of the data, you would have to expect a surprisingly large dissimilarity of interests within a clique over a certain account age based on this noise. Not 90%, but higher than the real value.

Sure, I expect data to be extremely noisy, but I am thinking about looking at very robust things (e.g. this guy is in rock-like perhaps-indie music), not making it too fine-grained.