The basic twitter system is easy enough. Now store all that data forever, run ads, collect ad stats, collect server stats, collect service stats, do authenticat

The basic twitter system is easy enough. Now store all that data forever, run ads, collect ad stats, collect server stats, collect service stats, do authentication for API calls, etc. etc. Things add up eventually. Suddenly your 6000 tweets/s has turned into a million op/s fanout. Problems become a lot less straightforward over network links too.