Building a software that one, two or even 100 people use at one time concurrently requires different set of skills than building a software that thousands or even millions of people use at one time. No disrespect to the Desktop software developers :), but building web applications especially in the nature of social network, requires focusing on scaling the applications to handle thousands concurrent requests at once. This is one the biggest challenges we are trying to deal with. Build a software that’s robust, well-designed but at the same time is scalable to handle large number of requests.
If you check out the Facebook engineering blog, you can find great tips regarding how to build software than can handle millions of concurrent requests.
Also facebook has open sourced a lot of tools they use that are great for scalability – check their open source home page at