Was rooting around for some information and stumbled across this page which may be of interest:
Namespaced Extensions in Feeds
Thursday, August 03, 2006
posted by Mihai Parparita
“I wrote a small MapReduce program to go over our BigTable and get the top 50 namespaces based on the number of feeds that use them.”
|% of Feeds||Namespace||URI|
Seems quite an impressive percentage for PRISM.