Are there any examples of company selling their social graph data in this manner directly?

The other companies I can think of (FB, Google, Tumblr) utilize the social graph to provide ad-targeting services, but don't charge to access the graph itself. The live nature of Twitter's firehose is probably better suited to sale as a recurring commodity than FB or Tumblr's.

I wonder if there's a price point where they could achieve widespread API usage while still making money with standard infrastructure (no custom datacenters, &c.)