* Google Reader has two kinds of feeds:
- feeds that have one subscriber (two thirds from the number of feeds, they’re updated every 3 hours)
- feeds that have more than one subscriber (these feeds are updated every hour)

* Google Reader uses 10 TB for storing all the raw data

* Google Reader crawls 8 million feeds

* Google Reader is the only major feed reader that keeps the entire history for all the feeds.

* many Google applications use Google Reader’s infrastructure for feeds: iGoogle, orkut, Gmail’s web clips, Blogger widgets, Google Spreadsheets, Ajax API. Google Reader is the place for any kind of user-driven activities that involve feeds and it’s independent from Google Blog Search.

* the rate of user growth = the rate of growth for the number of feeds

* the index size grows 4% every week

* 70% of the Google Reader traffic comes from Firefox (a lot of geeky users)

* Gmail and orkut are the only Google applications that have a bigger number of pageviews/user than Google Reader

* search requires a lot of computational resources. Google Reader uses two indexes for search:
- a big tree updated twice a day (150machines, 600 million documents)
- 40 small trees for recent posts, updated every 5 minutes (40 machines, 40million documents)

* future features:
- very soon: internationalization, feed recommendations, accepting pings sent to Google Blog Search
- in the near future: simple clustering based on links (posts that link to the same page), adding comments to the shared items
- idea for monetization: adding AdSense ads and sharing the revenue with publishers, assuming they use AdSense

Source: Google Operating System