Mark Granovetter,

the sociologist whose 1973 paper, The Strength of Weak Ties, has raised sociology's awareness on the existence of communites in social networks.

(from http://www.stanford.edu/dept/soc/granovet.html)
 

Duncan Watts & Steven Strogatz
the first to address the precence of clustering in real networks.

(from http://www.aip.org/aip/corporate/2000/watts.htm & http://tam.cornell.edu/Strogatz.html)
 

Jerry Grossman

Professor of Mathematics at Oakland University, the curator of Erdos number project.

(from http://www.oakland.edu/~grossman/)

The Erdos Number Project Page, maintained by Jerry Grossman at Oakland University.

http://www.oakland.edu/~grossman/erdoshp.html

Mark Newman,
who in Santa Fe Insitute was one of the first to study, using modern network ideas, the topology of the collaboration network of scientists.

(from http://www.santafe.edu/~mark/)

Tamas Vicsek

Professor of Physics at Eotvos University, in Budapest, the co-author of the paper addressing the evolution of the collaboration network on mathematicians and neuroscientists.

(from http://angel.elte.hu/~vicsek/)
 

Zoltan Neda

Professor of Physics at Babes-Bolyai University, Romania, and one of the authors of the study on network of mathematicians.

(from http://www.fi.uib.no/~neda/)
 

Erzebet Ravasz

graduate students at Univ. of Notre Dame, who has done a series of computer simulations on evolution of the collaboration networks.

(from http://www.nd.edu/~eravasz/)