Outbrain makes 100 billion recommendations per month as of October 2013.
Adam Rifkin stashed this in Content is king.
The company says it has been installed on more than 100,000 sites and has a network of 700 “premium publishers” that includes CNN, Hearst, Rolling Stone, and Fast Company. It also says it serves 100 billion recommendations each month, including both organic and paid recommendations. The company makes money by including sponsored content.
I am constantly impressed about the quality (=relevance, not necessarily the content itself) of the links they put forth.
Yes, they have some excellent algorithms for correlating relevance.