The most amazing thing about Instagram (some might say the only amazing thing) is that they have achived 30 million users. While many people will be trying to work out how to be the ‘next Instagram’, the more techy of us will be wondering how Instagram manages to support so many users.
As with Evernote, Instagram have also explained their backend. A key learning for startups is to not re-invent the wheel. Interestingly, they found that "Amazon’s network disk system (EBS) doesn’t support enough disk seeks per second", something I wrote about in early 2010. Just because a service is in the cloud it doesn’t mean it automatically scales. They are still real computers and you usually need to partition (shard) your data somehow.