I have written before that data around referral traffic could be a business model for both Twitter and Friendfeed. And as Twitter's growth accelerates, I believe this more than when I wrote the article one year ago. In fact, it is the reason that Bit.ly surpassed TinyURL so quickly: data. Bit.ly always provided metrics around clickthroughs - which proves to be a difficult measurement within Google Analytics (battle between referral traffic, direct traffic and the increasing usage of apps like Seesmic).
Tonight I noticed what appears to be a new set of metrics from Bit.ly: direct clicks and total clicks (what I am referring to as "pass throughs"). Retweet is a phenomenon on Twitter - but measurement is difficult because most people retweet with their own encoded URLs... consequently muddying the data and only representing a portion of the traffic. It is still incomplete data because it appears that Bit.ly is aggregating data based on the landing URL and it thus only represents activity for Bit.ly-encoded URLs... nevertheless, it is valuable for marketers in understanding the virality of shared content.
I still believe that richer analytics are valuable enough for marketers (who represent much of Twitter's activity) to represent a real subscription service... just look at how much businesses and marketers spend on sites like LinkedIn. Perhaps that means that Twitter acquires Bit.ly and expands on the analytics offering; or, perhaps it means that Twitter creates a service internally.