Market share of online payment processors?

What are the largest online payment processors in terms of market share and where can I find that kind of information (for free) ?

My personal guess is that Paypal and Authorize.net are the biggest in the small and medium business space, but I would like some hard data to confirm my estimate.

There was a link to a survey of payment processors for software vendors in this answer. There is a chart showing the popularity of payment processors in the survey.

You'll have to dig deeper. PayPal is owned by Ebay and Authorize by CyberSource Corporation, which is also a payment processor but for Enterprise-size businesses.