Esteban Arcaute Adam Kirsch Ravi Kumar David Liben-Nowell Sergei Vassilvitskii On threshold behavior in query incentive networks. 66-74 2007 conf/sigecom/2007 ACM Conference on Electronic Commerce http://doi.acm.org/10.1145/1250910.1250922 db/conf/sigecom/sigecom2007.html#ArcauteKKLV07