The MSigDB miRNA Motifs  ontology contains gene sets that share a 3'-UTR microRNA binding motif.
Statistics can now be found in the footers of each table in the GREAT web output.
All ontology data was downloaded and built between Oct 14, 2011 and Feb 18, 2012.
GREAT v1.8 MSigDB miRNA Motifs statistics are available here.
GREAT v1.2 MSigDB miRNA Motifs statistics are available here.
Subramanian, A., et al. Gene set enrichment analysis: a knowledge-based approach for interpreting genome-wide expression profiles, Proc Natl Acad Sci U S A. 2005 Oct 25; 102(43):15545-50.