Top Internet Startup Scalability Killers

It depends on what you do. Scalability is about building a system to handle ten fold more traffic than it's seeing now; <10 is too little, 100 is over-engineering (and nearly impossible).

Now imagine a graph data structure (any site with a news-feed/following functionality). You can get 10x increase in edges without a 10x increase vertices.

Some kind of products also increase in usefulness (and demand) as they increase in user growth, so solving scalability problems is necessary to their success. It isn't, however, sufficient.

Totally agree. I don't think anyone would have looked at LAMP 5 years ago and say it's scalable, but Facebook did it, in steps, because they had very talented engineers. I found the point of "don't use a relational database" ironic considering they contrasted Friendster and Facebook. Facebook uses MySQL as their backing store. While they might not use it in the traditional way, they most certainly evolved from joins and normalization to their current scheme as a sharded object store as it became critical to their growth.