Not quite sure how I came across this one exactly but I ended up going through some HTML source code and noted that amongst other things Twitter is using the Simple Storage Service (S3) from Amazon to host images. Used as an online storage web service; S3 allows for scalable online storage of a rather inordinate amount of data, running into Terrabytes. The service is charged on a Storage, Data Transfer and Requests tiered model and when you're talking about the sort of volume Twitter generates I guess it makes financial sense to use this service rather than fund your own hardware and bandwidth costs.

Looking at the details it seems fairly reasonable. For a 0.5 Terrabyte website, with 1TB of traffic p/month and a million requests p/day you'd run about ($60 + $170 + $300 respectively) $530 p/mnth running costs. On the surface this sounds alot, but compared to running your own servers and dealing with the associated bandwidth costs, this is pretty in-expensive.

There is a plethora of GUI based tools allowing almost FTP like interfaces with your data. It really absolves you from the cost-of-ownership and maintenance. I bet that's one seriously awesome looking data center !

Further Reading :