I think even defining what the market is, let alone who's got what share would be hard. Even harder than the slippery numbers that were dialup many years ago.
eg.
The number of megabits your customers generate?
The number of billable customers?
The number of domain names delegated to your name servers?
The number of IP addresses/servers/racks/etc?
ie. Hostworks probably don't have as many individual customers as, say, Webcentral but they have some big sites.
Where does Akamai fit in with delivery of sites such as Qantas, News.com.au, some of ABC etc?
Does Global Switch or Equinix count as part of the hosting market or not? Or just their customers?