This is a brilliant article. Are there other resources that go in as much depth as this article but aren't really stuck to RDBMS? What's the standard to doing graphs like these (linkedin/facebook)? RDBMS or a document oriented DB?
If you mean SQL specific resources, Joe Celko (who's book "SQL for Smarties" was mentioned in the article) has written a good deal. He generally expresses himself using ANSI Standard SQL (and served on the standards committees). But I think that the various NoSql solutions are far more popular for large scale implementations like LinkedIn/Facebook.
Yeah, thats what I thought that NoSql solutions like mongoDB etc. would be better suited for this purpose. However, I haven't come across a resource from NoSql side that goes in depth of explaining how to really implement: How you are connected with a certain person. etc.
Look at graph databases. These are very different from document stores or KV stores (NoSQL doesn't imply any commonality), though a KV store or something like HadoopFS might be used as a storage layer. They have their own challenges to scale up.