The HUGO Gene Nomenclature Committee (HGNC) Gene Families  ontology contains data on gene families based on sequence similarity, data from the literature and other databases, and manual curation.
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 HGNC Gene Families statistics are available here.
GREAT v1.2 HGNC Gene Families statistics are available here.
Bruford, E.A., The HGNC Database in 2008: a resource for the human genome, Nucleic Acids Res. 2008 Jan; 36(Database issue):D445-8.