User profiles and status messages are only parts of the puzzle of social networks. As a developer, you cannot go far without encountering file uploads, and they form a large portion of many types of enterprise applications, social networks included.
However, file uploads are a strange beast. The logic required to achieve them varies greatly from platform to platform. Some platforms, such as ColdFusion, handle file uploads almost implicitly. Other platforms, like ASP classic, are nightmarish when it comes to this area of development.
PHP, on the other hand, requires a bit of work, but after you have the hang of it, file uploads ...