These are well-known techniques in the telephony world. AT&T has been using this for many years to combat telecom fraud; knowing who you call means that if you don't pay your bill but another phone number starts calling people in your circle of friends, they can identify that it's you making those calls. Communities of interest have also been examined in the context of IP networks and email. It's an interesting field of research and this seems like a novel analysis, though I'm sure they are doing something very similar within every carrier network.

If you're interested in theory, read up on the DIMACS group at Rutgers. They are very well regarded.

UPenn, Cornell, Stanford immediate come to mind if you're looking to play at the top level. Look at proceedings of the POPL and PLDI conferences to get a sense of what people are doing and where they are doing it.

One good book on investing is David Chilton's The Wealthy Barber. I recommend it.